diff --git a/include/plasp/pddl/Action.h b/include/plasp/pddl/Action.h
index 4b618d9..d0b1b94 100644
--- a/include/plasp/pddl/Action.h
+++ b/include/plasp/pddl/Action.h
@@ -4,7 +4,7 @@
 #include <vector>
 
 #include <plasp/pddl/Expression.h>
-#include <plasp/pddl/Variable.h>
+#include <plasp/pddl/expressions/VariableExpression.h>
 #include <plasp/utils/Parser.h>
 
 namespace plasp
@@ -26,7 +26,7 @@ class Action
 	public:
 		const std::string &name() const;
 
-		const Variables &parameters() const;
+		const expressions::VariableExpressions &parameters() const;
 		const Expression &precondition() const;
 		const Expression &effect() const;
 
@@ -35,7 +35,7 @@ class Action
 
 		std::string m_name;
 
-		Variables m_parameters;
+		expressions::VariableExpressions m_parameters;
 		std::unique_ptr<Expression> m_precondition;
 		std::unique_ptr<Expression> m_effect;
 };
diff --git a/include/plasp/pddl/Expression.h b/include/plasp/pddl/Expression.h
index 0627c9d..1d054a7 100644
--- a/include/plasp/pddl/Expression.h
+++ b/include/plasp/pddl/Expression.h
@@ -22,6 +22,29 @@ namespace pddl
 class Context;
 class ExpressionVisitor;
 
+class Expression;
+using ExpressionPointer = std::unique_ptr<Expression>;
+using Expressions = std::vector<ExpressionPointer>;
+
+namespace expressions
+{
+class AndExpression;
+using AndExpressionPointer = std::unique_ptr<AndExpression>;
+
+class NotExpression;
+using NotExpressionPointer = std::unique_ptr<NotExpression>;
+
+class OrExpression;
+using OrExpressionPointer = std::unique_ptr<OrExpression>;
+
+class PredicateExpression;
+using PredicateExpressionPointer = std::unique_ptr<PredicateExpression>;
+
+class VariableExpression;
+using VariableExpressionPointer = std::unique_ptr<VariableExpression>;
+using VariableExpressions = std::vector<VariableExpressionPointer>;
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 class Expression
@@ -32,10 +55,13 @@ class Expression
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, Context &context, const Variables &parameters);
-std::unique_ptr<Expression> parseExpression(utils::Parser &parser, Context &context, const Variables &parameters);
+ExpressionPointer parsePreconditionExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters);
+ExpressionPointer parseExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters);
 
-std::unique_ptr<Expression> parseEffectExpression(utils::Parser &parser, Context &context, const Variables &parameters);
+ExpressionPointer parseEffectExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
diff --git a/include/plasp/pddl/expressions/AndExpression.h b/include/plasp/pddl/expressions/AndExpression.h
index c09f25e..2ce7ce6 100644
--- a/include/plasp/pddl/expressions/AndExpression.h
+++ b/include/plasp/pddl/expressions/AndExpression.h
@@ -20,7 +20,8 @@ class AndExpression: public NAryExpression
 {
 	public:
 		template<typename ExpressionParser>
-		static std::unique_ptr<AndExpression> parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression);
+		static AndExpressionPointer parse(utils::Parser &parser, Context &context,
+			const VariableExpressions &parameters, ExpressionParser parseExpression);
 
 	public:
 		void accept(ExpressionVisitor &expressionVisitor) const override;
@@ -32,7 +33,8 @@ class AndExpression: public NAryExpression
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 template<typename ExpressionParser>
-std::unique_ptr<AndExpression> AndExpression::parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression)
+AndExpressionPointer AndExpression::parse(utils::Parser &parser, Context &context,
+	const VariableExpressions &parameters, ExpressionParser parseExpression)
 {
 	auto expression = std::make_unique<AndExpression>(AndExpression());
 
diff --git a/include/plasp/pddl/expressions/NAryExpression.h b/include/plasp/pddl/expressions/NAryExpression.h
index 5574745..4008ba4 100644
--- a/include/plasp/pddl/expressions/NAryExpression.h
+++ b/include/plasp/pddl/expressions/NAryExpression.h
@@ -7,6 +7,7 @@
 
 #include <plasp/pddl/ConsistencyException.h>
 #include <plasp/pddl/Expression.h>
+#include <plasp/pddl/expressions/VariableExpression.h>
 #include <plasp/utils/Parser.h>
 
 namespace plasp
@@ -25,20 +26,22 @@ namespace expressions
 class NAryExpression: public Expression
 {
 	public:
-		const std::vector<std::unique_ptr<Expression>> &arguments() const;
+		const Expressions &arguments() const;
 
 	protected:
 		template<typename ExpressionParser>
-		void parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression);
+		void parse(utils::Parser &parser, Context &context, const VariableExpressions &parameters,
+			ExpressionParser parseExpression);
 
 	private:
-		std::vector<std::unique_ptr<Expression>> m_arguments;
+		Expressions m_arguments;
 };
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 template<typename ExpressionParser>
-void NAryExpression::parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression)
+void NAryExpression::parse(utils::Parser &parser, Context &context,
+	const VariableExpressions &parameters, ExpressionParser parseExpression)
 {
 	parser.skipWhiteSpace();
 
diff --git a/include/plasp/pddl/expressions/NotExpression.h b/include/plasp/pddl/expressions/NotExpression.h
index b2a8f84..982b1d8 100644
--- a/include/plasp/pddl/expressions/NotExpression.h
+++ b/include/plasp/pddl/expressions/NotExpression.h
@@ -21,7 +21,8 @@ class NotExpression: public Expression
 {
 	public:
 		template<typename ExpressionParser>
-		static std::unique_ptr<NotExpression> parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression);
+		static NotExpressionPointer parse(utils::Parser &parser, Context &context,
+			const VariableExpressions &parameters, ExpressionParser parseExpression);
 
 	public:
 		void accept(ExpressionVisitor &expressionVisitor) const override;
@@ -31,13 +32,14 @@ class NotExpression: public Expression
 	private:
 		NotExpression() = default;
 
-		std::unique_ptr<Expression> m_argument;
+		ExpressionPointer m_argument;
 };
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 template<typename ExpressionParser>
-std::unique_ptr<NotExpression> NotExpression::parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression)
+NotExpressionPointer NotExpression::parse(utils::Parser &parser, Context &context,
+	const VariableExpressions &parameters, ExpressionParser parseExpression)
 {
 	auto expression = std::make_unique<NotExpression>(NotExpression());
 
diff --git a/include/plasp/pddl/expressions/OrExpression.h b/include/plasp/pddl/expressions/OrExpression.h
index d234099..50f34ed 100644
--- a/include/plasp/pddl/expressions/OrExpression.h
+++ b/include/plasp/pddl/expressions/OrExpression.h
@@ -20,7 +20,8 @@ class OrExpression: public NAryExpression
 {
 	public:
 		template<typename ExpressionParser>
-		static std::unique_ptr<OrExpression> parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression);
+		static OrExpressionPointer parse(utils::Parser &parser, Context &context,
+			const VariableExpressions &parameters, ExpressionParser parseExpression);
 
 	public:
 		void accept(ExpressionVisitor &expressionVisitor) const override;
@@ -32,7 +33,8 @@ class OrExpression: public NAryExpression
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 template<typename ExpressionParser>
-std::unique_ptr<OrExpression> OrExpression::parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression)
+OrExpressionPointer OrExpression::parse(utils::Parser &parser, Context &context,
+	const VariableExpressions &parameters, ExpressionParser parseExpression)
 {
 	auto expression = std::make_unique<OrExpression>(OrExpression());
 
diff --git a/include/plasp/pddl/expressions/PredicateExpression.h b/include/plasp/pddl/expressions/PredicateExpression.h
index 219c6b5..2a30e1a 100644
--- a/include/plasp/pddl/expressions/PredicateExpression.h
+++ b/include/plasp/pddl/expressions/PredicateExpression.h
@@ -3,6 +3,7 @@
 
 #include <plasp/pddl/Expression.h>
 #include <plasp/pddl/Predicate.h>
+#include <plasp/pddl/expressions/VariableExpression.h>
 
 namespace plasp
 {
@@ -20,16 +21,19 @@ namespace expressions
 class PredicateExpression: public Expression
 {
 	public:
-		static std::unique_ptr<PredicateExpression> parse(std::string name, utils::Parser &parser, Context &context, const Variables &parameters);
+		static PredicateExpressionPointer parse(std::string name, utils::Parser &parser,
+			Context &context, const VariableExpressions &parameters);
 
 	public:
 		void accept(ExpressionVisitor &expressionVisitor) const override;
 
+		const std::vector<const VariableExpression *> &arguments() const;
+
 	private:
 		PredicateExpression() = default;
 
 		std::string m_name;
-		Variables m_arguments;
+		std::vector<const VariableExpression *> m_arguments;
 };
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/include/plasp/pddl/expressions/VariableExpression.h b/include/plasp/pddl/expressions/VariableExpression.h
new file mode 100644
index 0000000..a023036
--- /dev/null
+++ b/include/plasp/pddl/expressions/VariableExpression.h
@@ -0,0 +1,56 @@
+#ifndef __PLASP__PDDL__EXPRESSION__VARIABLE_EXPRESSION_H
+#define __PLASP__PDDL__EXPRESSION__VARIABLE_EXPRESSION_H
+
+#include <plasp/pddl/Expression.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// VariableExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+class VariableExpression: public Expression
+{
+	public:
+		static VariableExpressionPointer parseDeclaration(utils::Parser &parser);
+		static void parseTypedDeclaration(utils::Parser &parser, Context &context,
+			VariableExpressions &variableExpressions);
+
+		static const VariableExpression *parse(utils::Parser &parser,
+			const VariableExpressions &variableExpressions);
+
+	public:
+		void accept(ExpressionVisitor &expressionVisitor) const override;
+
+		const std::string &name() const;
+		TypePtr type() const;
+
+		void setDirty(bool isDirty = true);
+		bool isDirty() const;
+
+		void setType(TypePtr type);
+
+	private:
+		VariableExpression();
+
+		bool m_isDirty;
+
+		std::string m_name;
+
+		TypePtr m_type;
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}
+
+#endif
diff --git a/src/plasp/pddl/Action.cpp b/src/plasp/pddl/Action.cpp
index f58c163..a7e6e87 100644
--- a/src/plasp/pddl/Action.cpp
+++ b/src/plasp/pddl/Action.cpp
@@ -37,7 +37,7 @@ Action &Action::parseDeclaration(utils::Parser &parser, Context &context)
 	// Read parameters
 	while (parser.currentCharacter() != ')')
 	{
-		Variable::parseTyped(parser, context, action->m_parameters);
+		expressions::VariableExpression::parseTypedDeclaration(parser, context, action->m_parameters);
 
 		parser.skipWhiteSpace();
 	}
@@ -74,7 +74,7 @@ const std::string &Action::name() const
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-const Variables &Action::parameters() const
+const expressions::VariableExpressions &Action::parameters() const
 {
 	return m_parameters;
 }
diff --git a/src/plasp/pddl/Expression.cpp b/src/plasp/pddl/Expression.cpp
index 44de3b5..85dd902 100644
--- a/src/plasp/pddl/Expression.cpp
+++ b/src/plasp/pddl/Expression.cpp
@@ -19,9 +19,12 @@ namespace pddl
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parseExpressionContent(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters);
-std::unique_ptr<Expression> parseEffectBodyExpressionContent(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters);
-std::unique_ptr<Expression> parsePredicateExpression(utils::Parser &parser, Context &context, const Variables &parameters);
+ExpressionPointer parseExpressionContent(const std::string &expressionIdentifier,
+	utils::Parser &parser, Context &context, const expressions::VariableExpressions &parameters);
+ExpressionPointer parseEffectBodyExpressionContent(const std::string &expressionIdentifier,
+	utils::Parser &parser, Context &context, const expressions::VariableExpressions &parameters);
+ExpressionPointer parsePredicateExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
@@ -32,16 +35,20 @@ void throwUnsupported(const utils::Parser &parser, const std::string &expression
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parsePreconditionExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters)
 {
 	parser.expect<std::string>("(");
 
 	const auto expressionIdentifier = parser.parseIdentifier(isIdentifier);
 
-	std::unique_ptr<Expression> expression;
+	ExpressionPointer expression;
 
 	if (expressionIdentifier == "and")
-		expression = expressions::AndExpression::parse(parser, context, parameters, parsePreconditionExpression);
+	{
+		expression = expressions::AndExpression::parse(parser, context, parameters,
+			parsePreconditionExpression);
+	}
 	else if (expressionIdentifier == "forall"
 		|| expressionIdentifier == "preference")
 	{
@@ -57,7 +64,8 @@ std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, C
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parseExpression(utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parseExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters)
 {
 	parser.expect<std::string>("(");
 
@@ -72,11 +80,12 @@ std::unique_ptr<Expression> parseExpression(utils::Parser &parser, Context &cont
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parseExpressionContent(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parseExpressionContent(const std::string &expressionIdentifier,
+	utils::Parser &parser, Context &context, const expressions::VariableExpressions &parameters)
 {
 	parser.skipWhiteSpace();
 
-	std::unique_ptr<Expression> expression;
+	ExpressionPointer expression;
 
 	if (expressionIdentifier == "and")
 		expression = expressions::AndExpression::parse(parser, context, parameters, parseExpression);
@@ -122,13 +131,14 @@ std::unique_ptr<Expression> parseExpressionContent(const std::string &expression
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parseEffectExpression(utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parseEffectExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters)
 {
 	parser.expect<std::string>("(");
 
 	const auto expressionIdentifier = parser.parseIdentifier(isIdentifier);
 
-	std::unique_ptr<Expression> expression;
+	ExpressionPointer expression;
 
 	if (expressionIdentifier == "and")
 		expression = expressions::AndExpression::parse(parser, context, parameters, parseEffectExpression);
@@ -147,9 +157,10 @@ std::unique_ptr<Expression> parseEffectExpression(utils::Parser &parser, Context
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parseEffectBodyExpressionContent(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parseEffectBodyExpressionContent(const std::string &expressionIdentifier,
+	utils::Parser &parser, Context &context, const expressions::VariableExpressions &parameters)
 {
-	std::unique_ptr<Expression> expression;
+	ExpressionPointer expression;
 
 	if (expressionIdentifier == "not")
 		expression = expressions::NotExpression::parse(parser, context, parameters, parsePredicateExpression);
@@ -183,13 +194,14 @@ std::unique_ptr<Expression> parseEffectBodyExpressionContent(const std::string &
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<Expression> parsePredicateExpression(utils::Parser &parser, Context &context, const Variables &parameters)
+ExpressionPointer parsePredicateExpression(utils::Parser &parser, Context &context,
+	const expressions::VariableExpressions &parameters)
 {
 	parser.expect<std::string>("(");
 
 	const auto predicateName = parser.parseIdentifier(isIdentifier);
 
-	std::unique_ptr<Expression> expression;
+	ExpressionPointer expression;
 
 	// Check if predicate with that name exists
 	const auto match = std::find_if(context.predicates.cbegin(), context.predicates.cend(),
diff --git a/src/plasp/pddl/expressions/PredicateExpression.cpp b/src/plasp/pddl/expressions/PredicateExpression.cpp
index 88d3450..8a17e38 100644
--- a/src/plasp/pddl/expressions/PredicateExpression.cpp
+++ b/src/plasp/pddl/expressions/PredicateExpression.cpp
@@ -15,7 +15,8 @@ namespace expressions
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::unique_ptr<PredicateExpression> PredicateExpression::parse(std::string name, utils::Parser &parser, Context &context, const Variables &parameters)
+PredicateExpressionPointer PredicateExpression::parse(std::string name, utils::Parser &parser,
+	Context &context, const VariableExpressions &parameters)
 {
 	auto expression = std::make_unique<PredicateExpression>(PredicateExpression());
 
@@ -25,7 +26,7 @@ std::unique_ptr<PredicateExpression> PredicateExpression::parse(std::string name
 
 	// Parse arguments
 	while (parser.currentCharacter() != ')')
-		Variable::parseTyped(parser, context, expression->m_arguments);
+		expression->m_arguments.emplace_back(VariableExpression::parse(parser, parameters));
 
 	// TODO: check that signature matches one of the declared ones
 
@@ -41,6 +42,13 @@ void PredicateExpression::accept(plasp::pddl::ExpressionVisitor &expressionVisit
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+const std::vector<const VariableExpression *> &PredicateExpression::arguments() const
+{
+	return m_arguments;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 }
 }
 }
diff --git a/src/plasp/pddl/expressions/VariableExpression.cpp b/src/plasp/pddl/expressions/VariableExpression.cpp
new file mode 100644
index 0000000..c578113
--- /dev/null
+++ b/src/plasp/pddl/expressions/VariableExpression.cpp
@@ -0,0 +1,142 @@
+#include <plasp/pddl/expressions/VariableExpression.h>
+
+#include <algorithm>
+
+#include <plasp/pddl/Context.h>
+#include <plasp/pddl/ExpressionVisitor.h>
+#include <plasp/pddl/Identifier.h>
+#include <plasp/utils/ParserException.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// VariableExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+VariableExpression::VariableExpression()
+:	m_isDirty{false}
+{
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+VariableExpressionPointer VariableExpression::parseDeclaration(utils::Parser &parser)
+{
+	parser.skipWhiteSpace();
+
+	parser.expect<std::string>("?");
+
+	auto variable = std::make_unique<VariableExpression>(VariableExpression());
+
+	variable->m_name = parser.parseIdentifier(isIdentifier);
+	variable->setDirty();
+
+	return variable;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void VariableExpression::parseTypedDeclaration(utils::Parser &parser, Context &context,
+	VariableExpressions &variableExpressions)
+{
+	// Parse and store variable itself
+	variableExpressions.emplace_back(parseDeclaration(parser));
+
+	parser.skipWhiteSpace();
+
+	// Check if the variable has a type declaration
+	if (!parser.advanceIf('-'))
+		return;
+
+	// Parse argument type
+	const auto type = parseType(parser, context);
+
+	// Set the argument type for all previously flagged arguments
+	std::for_each(variableExpressions.begin(), variableExpressions.end(),
+		[&](auto &variable)
+		{
+			if (!variable->isDirty())
+				return;
+
+			variable->setType(type);
+			variable->setDirty(false);
+		});
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+const VariableExpression *VariableExpression::parse(utils::Parser &parser,
+	const VariableExpressions &variableExpressions)
+{
+	parser.skipWhiteSpace();
+
+	parser.expect<std::string>("?");
+
+	const auto variableName = parser.parseIdentifier(isIdentifier);
+
+	const auto match = std::find_if(variableExpressions.cbegin(), variableExpressions.cend(),
+		[&](const auto &variableExpression)
+		{
+			return variableExpression->name() == variableName;
+		});
+
+	if (match == variableExpressions.cend())
+		throw utils::ParserException(parser.row(), parser.column(), "Variable \"" + variableName + "\" used but never declared");
+
+	return match->get();
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void VariableExpression::accept(plasp::pddl::ExpressionVisitor &expressionVisitor) const
+{
+	expressionVisitor.visit(*this);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+const std::string &VariableExpression::name() const
+{
+	return m_name;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+TypePtr VariableExpression::type() const
+{
+	return m_type;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void VariableExpression::setDirty(bool isDirty)
+{
+	m_isDirty = isDirty;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+bool VariableExpression::isDirty() const
+{
+	return m_isDirty;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void VariableExpression::setType(TypePtr type)
+{
+	m_type = type;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}