diff --git a/include/plasp/pddl/Expression.h b/include/plasp/pddl/Expression.h
index 481093b..3c1879c 100644
--- a/include/plasp/pddl/Expression.h
+++ b/include/plasp/pddl/Expression.h
@@ -33,6 +33,7 @@ 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);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
diff --git a/include/plasp/pddl/expressions/AndExpression.h b/include/plasp/pddl/expressions/AndExpression.h
index 924d96e..c09f25e 100644
--- a/include/plasp/pddl/expressions/AndExpression.h
+++ b/include/plasp/pddl/expressions/AndExpression.h
@@ -39,7 +39,7 @@ std::unique_ptr<AndExpression> AndExpression::parse(utils::Parser &parser, Conte
 	expression->NAryExpression::parse(parser, context, parameters, parseExpression);
 
 	if (expression->arguments().empty())
-		throw ConsistencyException("Expressions should not be empty");
+		throw ConsistencyException("\"and\" expressions should not be empty");
 
 	return expression;
 }
diff --git a/include/plasp/pddl/expressions/OrExpression.h b/include/plasp/pddl/expressions/OrExpression.h
new file mode 100644
index 0000000..d234099
--- /dev/null
+++ b/include/plasp/pddl/expressions/OrExpression.h
@@ -0,0 +1,53 @@
+#ifndef __PLASP__PDDL__EXPRESSION__OR_EXPRESSION_H
+#define __PLASP__PDDL__EXPRESSION__OR_EXPRESSION_H
+
+#include <plasp/pddl/expressions/NAryExpression.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// OrExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+class OrExpression: public NAryExpression
+{
+	public:
+		template<typename ExpressionParser>
+		static std::unique_ptr<OrExpression> parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression);
+
+	public:
+		void accept(ExpressionVisitor &expressionVisitor) const override;
+
+	private:
+		OrExpression() = default;
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+template<typename ExpressionParser>
+std::unique_ptr<OrExpression> OrExpression::parse(utils::Parser &parser, Context &context, const Variables &parameters, ExpressionParser parseExpression)
+{
+	auto expression = std::make_unique<OrExpression>(OrExpression());
+
+	expression->NAryExpression::parse(parser, context, parameters, parseExpression);
+
+	if (expression->arguments().empty())
+		throw ConsistencyException("\"or\" expressions should not be empty");
+
+	return expression;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}
+
+#endif
diff --git a/include/plasp/pddl/expressions/PredicateExpression.h b/include/plasp/pddl/expressions/PredicateExpression.h
new file mode 100644
index 0000000..8381284
--- /dev/null
+++ b/include/plasp/pddl/expressions/PredicateExpression.h
@@ -0,0 +1,42 @@
+#ifndef __PLASP__PDDL__EXPRESSION__PREDICATE_EXPRESSION_H
+#define __PLASP__PDDL__EXPRESSION__PREDICATE_EXPRESSION_H
+
+#include <plasp/pddl/Expression.h>
+#include <plasp/pddl/Identifier.h>
+#include <plasp/pddl/Predicate.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// PredicateExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+class PredicateExpression: public Expression
+{
+	public:
+		static std::unique_ptr<PredicateExpression> parse(std::string name, utils::Parser &parser, Context &context, const Variables &parameters);
+
+	public:
+		void accept(ExpressionVisitor &expressionVisitor) const override;
+
+	private:
+		PredicateExpression() = default;
+
+		std::string m_name;
+		Variables m_arguments;
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}
+
+#endif
diff --git a/src/plasp/pddl/Action.cpp b/src/plasp/pddl/Action.cpp
index 24113c9..cbf7927 100644
--- a/src/plasp/pddl/Action.cpp
+++ b/src/plasp/pddl/Action.cpp
@@ -4,6 +4,7 @@
 
 #include <plasp/pddl/Context.h>
 #include <plasp/pddl/Identifier.h>
+#include <plasp/utils/ParserException.h>
 
 namespace plasp
 {
@@ -52,7 +53,8 @@ Action &Action::parseDeclaration(utils::Parser &parser, Context &context)
 
 		if (sectionIdentifier == "precondition")
 			action->m_precondition = parsePreconditionExpression(parser, context, action->m_parameters);
-		//else if (sectionIdentifier == "effect")
+		else if (sectionIdentifier == "effect")
+			throw utils::ParserException(parser.row(), parser.column(), "Action effects are currently unsupported");
 		//	action->m_effect = parseEffectExpression(parser, context);
 	}
 
diff --git a/src/plasp/pddl/Expression.cpp b/src/plasp/pddl/Expression.cpp
index 0bf0b90..d55efee 100644
--- a/src/plasp/pddl/Expression.cpp
+++ b/src/plasp/pddl/Expression.cpp
@@ -3,6 +3,8 @@
 #include <plasp/pddl/Context.h>
 #include <plasp/pddl/Identifier.h>
 #include <plasp/pddl/expressions/AndExpression.h>
+#include <plasp/pddl/expressions/OrExpression.h>
+#include <plasp/pddl/expressions/PredicateExpression.h>
 #include <plasp/utils/ParserException.h>
 
 namespace plasp
@@ -16,6 +18,10 @@ namespace pddl
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+std::unique_ptr<Expression> parseExpressionBody(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters);
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, Context &context, const Variables &parameters)
 {
 	parser.skipWhiteSpace();
@@ -24,26 +30,24 @@ std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, C
 
 	const auto expressionIdentifier = parser.parseIdentifier(isIdentifier);
 
-	std::cout << "Parsing identifier " << expressionIdentifier << std::endl;
+	std::cout << "Parsing precondition expression " << expressionIdentifier << std::endl;
 
 	std::unique_ptr<Expression> expression;
 
-	const auto throwNotAllowed =
+	const auto throwUnsupported =
 		[&]()
 		{
-			throw utils::ParserException(parser.row(), parser.column(), "Expression of type \"" + expressionIdentifier + "\" not allowed in preference declaration");
+			throw utils::ParserException(parser.row(), parser.column(), "Expression type \"" + expressionIdentifier + "\" unsupported");
 		};
 
 	if (expressionIdentifier == "and")
 		expression = expressions::AndExpression::parse(parser, context, parameters, parsePreconditionExpression);
-	else if (expressionIdentifier == "or")
-		throwNotAllowed();
-	else if (expressionIdentifier == "not")
-		throwNotAllowed();
-	else if (expressionIdentifier == "exists")
-		throwNotAllowed();
+	else if (expressionIdentifier == "forall")
+		throwUnsupported();
+	else if (expressionIdentifier == "preference")
+		throwUnsupported();
 	else
-		throw utils::ParserException(parser.row(), parser.column(), "Undeclared expression \"" + expressionIdentifier + "\"");
+		expression = parseExpressionBody(expressionIdentifier, parser, context, parameters);
 
 	parser.expect<std::string>(")");
 
@@ -52,5 +56,79 @@ std::unique_ptr<Expression> parsePreconditionExpression(utils::Parser &parser, C
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+std::unique_ptr<Expression> parseExpression(utils::Parser &parser, Context &context, const Variables &parameters)
+{
+	parser.skipWhiteSpace();
+
+	parser.expect<std::string>("(");
+
+	const auto expressionIdentifier = parser.parseIdentifier(isIdentifier);
+
+	auto expression = parseExpressionBody(expressionIdentifier, parser, context, parameters);
+
+	parser.expect<std::string>(")");
+
+	return expression;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::unique_ptr<Expression> parseExpressionBody(const std::string &expressionIdentifier, utils::Parser &parser, Context &context, const Variables &parameters)
+{
+	parser.skipWhiteSpace();
+
+	std::cout << "Parsing expression " << expressionIdentifier << std::endl;
+
+	std::unique_ptr<Expression> expression;
+
+	const auto throwUnsupported =
+	[&]()
+	{
+		throw utils::ParserException(parser.row(), parser.column(), "Expression type \"" + expressionIdentifier + "\" unsupported");
+	};
+
+	if (expressionIdentifier == "and")
+		expression = expressions::AndExpression::parse(parser, context, parameters, parseExpression);
+	else if (expressionIdentifier == "or")
+		expression = expressions::OrExpression::parse(parser, context, parameters, parseExpression);
+	else if (expressionIdentifier == "not"
+		|| expressionIdentifier == "imply"
+		|| expressionIdentifier == "exists"
+		|| expressionIdentifier == "forall"
+		|| expressionIdentifier == "-"
+		|| expressionIdentifier == "="
+		|| expressionIdentifier == "*"
+		|| expressionIdentifier == "+"
+		|| expressionIdentifier == "-"
+		|| expressionIdentifier == "/"
+		|| expressionIdentifier == ">"
+		|| expressionIdentifier == "<"
+		|| expressionIdentifier == "="
+		|| expressionIdentifier == ">="
+		|| expressionIdentifier == "<=")
+	{
+		throwUnsupported();
+	}
+	else
+	{
+		// Check if predicate with that name exists
+		const auto match = std::find_if(context.predicates.cbegin(), context.predicates.cend(),
+			[&](const auto &predicate)
+			{
+				return predicate->name() == expressionIdentifier;
+			});
+
+		// If predicate exists, parse it
+		if (match != context.predicates.cend())
+			expression = expressions::PredicateExpression::parse(expressionIdentifier, parser, context, parameters);
+		else
+			throw utils::ParserException(parser.row(), parser.column(), "Undeclared expression \"" + expressionIdentifier + "\"");
+	}
+
+	return expression;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 }
 }
diff --git a/src/plasp/pddl/expressions/AndExpression.cpp b/src/plasp/pddl/expressions/AndExpression.cpp
index 5dddbac..0b274c9 100644
--- a/src/plasp/pddl/expressions/AndExpression.cpp
+++ b/src/plasp/pddl/expressions/AndExpression.cpp
@@ -1,8 +1,6 @@
 #include <plasp/pddl/expressions/AndExpression.h>
 
-#include <plasp/pddl/Context.h>
 #include <plasp/pddl/ExpressionVisitor.h>
-#include <plasp/utils/Parser.h>
 
 namespace plasp
 {
diff --git a/src/plasp/pddl/expressions/OrExpression.cpp b/src/plasp/pddl/expressions/OrExpression.cpp
new file mode 100644
index 0000000..428e4aa
--- /dev/null
+++ b/src/plasp/pddl/expressions/OrExpression.cpp
@@ -0,0 +1,27 @@
+#include <plasp/pddl/expressions/OrExpression.h>
+
+#include <plasp/pddl/ExpressionVisitor.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// OrExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void OrExpression::accept(plasp::pddl::ExpressionVisitor &expressionVisitor) const
+{
+	expressionVisitor.visit(*this);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}
diff --git a/src/plasp/pddl/expressions/PredicateExpression.cpp b/src/plasp/pddl/expressions/PredicateExpression.cpp
new file mode 100644
index 0000000..45c7c37
--- /dev/null
+++ b/src/plasp/pddl/expressions/PredicateExpression.cpp
@@ -0,0 +1,46 @@
+#include <plasp/pddl/expressions/PredicateExpression.h>
+
+#include <plasp/pddl/ExpressionVisitor.h>
+
+namespace plasp
+{
+namespace pddl
+{
+namespace expressions
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// PredicateExpression
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::unique_ptr<PredicateExpression> PredicateExpression::parse(std::string name, utils::Parser &parser, Context &context, const Variables &parameters)
+{
+	auto expression = std::make_unique<PredicateExpression>(PredicateExpression());
+
+	expression->m_name = name;
+
+	parser.skipWhiteSpace();
+
+	// Parse arguments
+	while (parser.currentCharacter() != ')')
+		Variable::parseTyped(parser, context, expression->m_arguments);
+
+	std::cout << "Parsed " << expression->m_arguments.size() << " arguments" << std::endl;
+
+	return expression;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+void PredicateExpression::accept(plasp::pddl::ExpressionVisitor &expressionVisitor) const
+{
+	expressionVisitor.visit(*this);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+}