diff --git a/lib/pddlparse/include/pddlparse/ASTForward.h b/lib/pddlparse/include/pddlparse/ASTForward.h
index 6a520b5..56b98d0 100644
--- a/lib/pddlparse/include/pddlparse/ASTForward.h
+++ b/lib/pddlparse/include/pddlparse/ASTForward.h
@@ -241,11 +241,14 @@ using Literals = std::vector<Literal>;
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+class Fact;
+
 namespace detail
 {
 using FactT = Variant<
 	AtomicFormula,
-	AtPointer<Literal>>;
+	AtPointer<Literal>,
+	NotPointer<Fact>>;
 }
 
 class Fact : public detail::FactT
diff --git a/lib/pddlparse/include/pddlparse/detail/parsing/Fact.h b/lib/pddlparse/include/pddlparse/detail/parsing/Fact.h
new file mode 100644
index 0000000..e55b076
--- /dev/null
+++ b/lib/pddlparse/include/pddlparse/detail/parsing/Fact.h
@@ -0,0 +1,27 @@
+#ifndef __PDDL_PARSE__DETAIL__PARSING__FACT_H
+#define __PDDL_PARSE__DETAIL__PARSING__FACT_H
+
+#include <pddlparse/ASTForward.h>
+#include <pddlparse/Context.h>
+#include <pddlparse/detail/ASTContext.h>
+#include <pddlparse/detail/VariableStack.h>
+
+namespace pddl
+{
+namespace detail
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// Fact
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::experimental::optional<ast::Fact> parseFact(Context &context, ASTContext &astContext, VariableStack &variableStack);
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+
+#endif
diff --git a/lib/pddlparse/include/pddlparse/detail/parsing/InitialState.h b/lib/pddlparse/include/pddlparse/detail/parsing/InitialState.h
index 10afbe2..b4c41a9 100644
--- a/lib/pddlparse/include/pddlparse/detail/parsing/InitialState.h
+++ b/lib/pddlparse/include/pddlparse/detail/parsing/InitialState.h
@@ -15,7 +15,7 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-ast::InitialState parseInitialState(Context &context, ASTContext &expressionContext);
+ast::InitialState parseInitialState(Context &context, ASTContext &astContext, VariableStack &variableStack);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/Fact.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/Fact.cpp
new file mode 100644
index 0000000..02806a4
--- /dev/null
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/Fact.cpp
@@ -0,0 +1,53 @@
+#include <pddlparse/detail/parsing/Fact.h>
+
+#include <pddlparse/AST.h>
+#include <pddlparse/detail/parsing/AtomicFormula.h>
+#include <pddlparse/detail/parsing/Expressions.h>
+#include <pddlparse/detail/parsing/Unsupported.h>
+
+namespace pddl
+{
+namespace detail
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// Fact
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::experimental::optional<ast::Fact> parseFact(Context &context, ASTContext &astContext, VariableStack &variableStack)
+{
+	auto &tokenizer = context.tokenizer;
+
+	// Test unsupported expressions first
+	const auto position = tokenizer.position();
+
+	tokenizer.expect<std::string>("(");
+	tokenizer.skipWhiteSpace();
+
+	if (tokenizer.testIdentifierAndReturn("=")
+		|| tokenizer.testIdentifierAndReturn("at"))
+	{
+		tokenizer.seek(position);
+		return parseUnsupported(context);
+	}
+
+	tokenizer.seek(position);
+
+	// Now, test supported expressions
+	std::experimental::optional<ast::Fact> fact;
+
+	if ((fact = parseNot<ast::Fact>(context, astContext, variableStack, parseAtomicFormula))
+	    || (fact = parseAtomicFormula(context, astContext, variableStack)))
+	{
+		return std::move(fact.value());
+	}
+
+	return parseAtomicFormula(context, astContext, variableStack);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/InitialState.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/InitialState.cpp
index 362969e..fdfa7c3 100644
--- a/lib/pddlparse/src/pddlparse/detail/parsing/InitialState.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/InitialState.cpp
@@ -2,6 +2,7 @@
 
 #include <pddlparse/AST.h>
 #include <pddlparse/ParserException.h>
+#include <pddlparse/detail/parsing/Fact.h>
 
 namespace pddl
 {
@@ -14,62 +15,25 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-ast::InitialState parseInitialState(Context &context, ASTContext &)
+ast::InitialState parseInitialState(Context &context, ASTContext &astContext, VariableStack &variableStack)
 {
 	auto &tokenizer = context.tokenizer;
 
 	ast::InitialState initialState;
 
-	context.warningCallback(tokenizer.location(), "initial state parser under construction, section is currently ignored");
-
-	// TODO: reimplement
-	/*const auto parseInitialStateElement =
-		[&]() -> ExpressionPointer
-		{
-			ExpressionPointer expression;
-
-			// TODO: do not allow negative initial state literals
-			if ((expression = parseLiteral(context, expressionContext))
-				|| (expression = expressions::At::parse(context, expressionContext, parseLiteral)))
-			{
-				return expression;
-			}
-
-			const auto position = tokenizer.position();
-
-			tokenizer.expect<std::string>("(");
-
-			const auto expressionIdentifierPosition = tokenizer.position();
-
-			if (tokenizer.testIdentifierAndSkip("="))
-			{
-				tokenizer.seek(expressionIdentifierPosition);
-				const auto expressionIdentifier = tokenizer.getIdentifier();
-
-				tokenizer.seek(position);
-				return expressions::Unsupported::parse(context);
-			}
-
-			tokenizer.seek(expressionIdentifierPosition);
-			const auto expressionIdentifier = tokenizer.getIdentifier();
-
-			tokenizer.seek(position);
-			throw tokenize::TokenizerException(tokenizer.location(), "expression type “" + expressionIdentifier + "” unknown or not allowed in this context");
-		};
-
 	tokenizer.skipWhiteSpace();
 
 	while (tokenizer.currentCharacter() != ')')
 	{
-		ast::Expression expression;
+		auto fact = parseFact(context, astContext, variableStack);
 
-		if ((expression = parseInitialStateElement()))
-			initialState->m_facts.emplace_back(std::move(expression));
+		if (!fact)
+			throw ParserException(tokenizer.location(), "invalid initial state fact");
+
+		initialState.facts.emplace_back(std::move(fact.value()));
 
 		tokenizer.skipWhiteSpace();
-	}*/
-
-	skipSection(tokenizer);
+	}
 
 	return initialState;
 }
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/Problem.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/Problem.cpp
index 21709df..b307cdb 100644
--- a/lib/pddlparse/src/pddlparse/detail/parsing/Problem.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/Problem.cpp
@@ -267,12 +267,10 @@ void ProblemParser::parseInitialStateSection(ast::Problem &problem)
 	tokenizer.expect<std::string>("init");
 
 	ASTContext astContext(problem);
+	VariableStack variableStack;
 
-	m_context.warningCallback(tokenizer.location(), "initial state parser under construction, section is currently ignored");
-
-	// TODO: reimplement
-	//problem.initialState = parseInitialState(m_context, astContext);
-	//tokenizer.expect<std::string>(")");
+	problem.initialState = parseInitialState(m_context, astContext, variableStack);
+	tokenizer.expect<std::string>(")");
 
 	skipSection(tokenizer);
 }