diff --git a/src/Hl7.Fhir.Base/FhirPath/EvaluationContext.cs b/src/Hl7.Fhir.Base/FhirPath/EvaluationContext.cs
index 36a1af6241..35884e22f5 100644
--- a/src/Hl7.Fhir.Base/FhirPath/EvaluationContext.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/EvaluationContext.cs
@@ -29,6 +29,11 @@ public EvaluationContext(ITypedElement resource, ITypedElement rootResource)
             Resource = resource;
             RootResource = rootResource ?? resource;
         }
+        
+        public EvaluationContext(ITypedElement resource, ITypedElement rootResource, IDictionary<string, IEnumerable<ITypedElement>> environment) : this(resource, rootResource)
+        {
+            Environment = environment;
+        }
 
         /// <summary>
         /// The data represented by <c>%rootResource</c>.
@@ -39,6 +44,11 @@ public EvaluationContext(ITypedElement resource, ITypedElement rootResource)
         /// The data represented by <c>%resource</c>.
         /// </summary>
         public ITypedElement Resource { get; set; }
+        
+        /// <summary>
+        /// The environment variables that are available to the FHIRPath expressions.
+        /// </summary>
+        public IDictionary<string, IEnumerable<ITypedElement>> Environment { get; set; }
 
         /// <summary>
         /// A delegate that handles the output for the <c>trace()</c> function.
diff --git a/src/Hl7.Fhir.Base/FhirPath/Expressions/EvaluatorVisitor.cs b/src/Hl7.Fhir.Base/FhirPath/Expressions/EvaluatorVisitor.cs
index 5c0ed26cd5..19a675ca85 100644
--- a/src/Hl7.Fhir.Base/FhirPath/Expressions/EvaluatorVisitor.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/Expressions/EvaluatorVisitor.cs
@@ -80,6 +80,11 @@ public override Invokee VisitVariableRef(FP.VariableRefExpression expression)
             if (expression.Name == "rootResource")
                 return InvokeeFactory.GetRootResource;
 
+            if (expression is ContextVariableRefExpression Cvre)
+            {
+                return Cvre.Resolve;
+            }
+
             // Variables are still functions without arguments. For now variables are treated separately here,
             //Functions are handled elsewhere.
             return resolve(Symbols, expression.Name, Enumerable.Empty<Type>());
diff --git a/src/Hl7.Fhir.Base/FhirPath/Expressions/ExpressionNode.cs b/src/Hl7.Fhir.Base/FhirPath/Expressions/ExpressionNode.cs
index fd84567cdc..98b9718dc9 100644
--- a/src/Hl7.Fhir.Base/FhirPath/Expressions/ExpressionNode.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/Expressions/ExpressionNode.cs
@@ -329,6 +329,14 @@ public override int GetHashCode()
             return base.GetHashCode() ^ Name.GetHashCode();
         }
     }
+    
+    public class ContextVariableRefExpression(string name) : VariableRefExpression(name)
+    {
+        internal IEnumerable<ITypedElement> Resolve(Closure context, IEnumerable<Invokee> _)
+        {
+            return context.EvaluationContext.Environment[Name] ?? throw Error.InvalidOperation($"Variable {Name} not found in environment");
+        }
+    }
 
     public class AxisExpression : VariableRefExpression
     {
diff --git a/src/Hl7.Fhir.Base/FhirPath/FhirEvaluationContext.cs b/src/Hl7.Fhir.Base/FhirPath/FhirEvaluationContext.cs
index cb1c552171..f26d955f18 100644
--- a/src/Hl7.Fhir.Base/FhirPath/FhirEvaluationContext.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/FhirEvaluationContext.cs
@@ -10,6 +10,7 @@
 using Hl7.Fhir.Specification.Terminology;
 using Hl7.FhirPath;
 using System;
+using System.Collections.Generic;
 
 namespace Hl7.Fhir.FhirPath
 {
@@ -33,6 +34,15 @@ public FhirEvaluationContext(ITypedElement resource, ITypedElement rootResource)
         {
         }
 
+        /// <summary>
+        /// Create a FhirEvaluationContext with a resource and an environment.
+        /// </summary>
+        /// <param name="resource"></param>
+        /// <param name="environment"></param>
+        public FhirEvaluationContext(ITypedElement resource, IDictionary<string, IEnumerable<ITypedElement>> environment) : base(resource, null, environment)
+        {
+        }
+
         /// <summary>
         /// Create a FhirEvaluationContext and also set the variables <c>%resource</c> and <c>%rootResource</c> to their correct values.
         /// </summary>
diff --git a/src/Hl7.Fhir.Base/FhirPath/Parser/Grammar.cs b/src/Hl7.Fhir.Base/FhirPath/Parser/Grammar.cs
index 439b0228a2..74b96e72cd 100644
--- a/src/Hl7.Fhir.Base/FhirPath/Parser/Grammar.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/Parser/Grammar.cs
@@ -60,6 +60,7 @@ internal class Grammar
         //  : invocation                                            #invocationTerm
         //  | literal                                               #literalTerm
         //  | externalConstant                                      #externalConstantTerm
+        //  | externalVariable                                      #externalVariableTerm
         //  | '(' expression ')'                                    #parenthesizedTerm
         //  | '{' '}'                                               #nullLiteral
         //  ;
@@ -101,7 +102,8 @@ public static Parser<Expression> FunctionInvocation(Expression focus)
         public static readonly Parser<Expression> Term =
             Literal
             .Or(FunctionInvocation(AxisExpression.That))
-            .XOr(Lexer.ExternalConstant.Select(n => BuildVariableRefExpression(n))) //Was .XOr(Lexer.ExternalConstant.Select(v => Eval.ExternalConstant(v)))
+            .XOr(Lexer.ExternalVariable.Select(n => new ContextVariableRefExpression(n)))
+            .Or(Lexer.ExternalConstant.Select(n => BuildVariableRefExpression(n))) //Was .XOr(Lexer.ExternalConstant.Select(v => Eval.ExternalConstant(v)))
             .XOr(BracketExpr)
             .XOr(EmptyList)
             .XOr(Lexer.Axis.Select(a => new AxisExpression(a)))
diff --git a/src/Hl7.Fhir.Base/FhirPath/Parser/Lexer.cs b/src/Hl7.Fhir.Base/FhirPath/Parser/Lexer.cs
index 47c69bc53f..b412267b4b 100644
--- a/src/Hl7.Fhir.Base/FhirPath/Parser/Lexer.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/Parser/Lexer.cs
@@ -78,12 +78,19 @@ from closeQ in Parse.Char(delimiter)
         //  ;
         public static readonly Parser<string> Identifier =
             Id.XOr(DelimitedIdentifier);
+        
+        // externalVariable
+        //  : '%%' identifier
+        //  ;
+        public static readonly Parser<string> ExternalVariable =
+            Parse.String("%%").Then(_ => Identifier.XOr(String))
+                .Named("external variable");
 
         // externalConstant
         //  : '%' identifier
         //  ;
         public static readonly Parser<string> ExternalConstant =
-            Parse.Char('%').Then(c => Identifier.XOr(String))
+            Parse.Char('%').Then(_ => Identifier.XOr(String))
             .Named("external constant");
 
         // DATE
diff --git a/src/Hl7.FhirPath.Tests/Tests/EnviromentTests.cs b/src/Hl7.FhirPath.Tests/Tests/EnviromentTests.cs
new file mode 100644
index 0000000000..8901e1e387
--- /dev/null
+++ b/src/Hl7.FhirPath.Tests/Tests/EnviromentTests.cs
@@ -0,0 +1,20 @@
+using Hl7.Fhir.ElementModel;
+using Hl7.FhirPath;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.Collections.Generic;
+
+namespace HL7.FhirPath.Tests.Tests;
+
+[TestClass]
+public class EnviromentTests
+{
+    [TestMethod]
+    public void TestEnvironment()
+    {
+        var compiler = new FhirPathCompiler();
+        var expr = compiler.Compile("%%var = 1");
+        
+        expr.IsTrue(null, new EvaluationContext(null, null, new Dictionary<string, IEnumerable<ITypedElement>> { { "var", new [] { ElementNode.ForPrimitive(1) } } }));
+        expr.IsBoolean(false, null, new EvaluationContext(null, null, new Dictionary<string, IEnumerable<ITypedElement>> { { "var", new[] { ElementNode.ForPrimitive(2) } } }));
+    }
+}
\ No newline at end of file
diff --git a/src/Hl7.FhirPath.Tests/Tests/FhirPathGrammarTest.cs b/src/Hl7.FhirPath.Tests/Tests/FhirPathGrammarTest.cs
index 499fd98305..c76e11504c 100644
--- a/src/Hl7.FhirPath.Tests/Tests/FhirPathGrammarTest.cs
+++ b/src/Hl7.FhirPath.Tests/Tests/FhirPathGrammarTest.cs
@@ -75,6 +75,7 @@ public void FhirPath_Gramm_Term()
             AssertParser.SucceedsMatch(parser, "doSomething('hi', 3.14)", new FunctionCallExpression(AxisExpression.This, "doSomething", TypeSpecifier.Any,
                         new ConstantExpression("hi"), new ConstantExpression(3.14m)));
             AssertParser.SucceedsMatch(parser, "%external", new VariableRefExpression("external"));
+            AssertParser.SucceedsMatch(parser, "%%contextvar", new ContextVariableRefExpression("contextvar"));
             AssertParser.SucceedsMatch(parser, "@2013-12", new ConstantExpression(P.Date.Parse("2013-12")));
             AssertParser.SucceedsMatch(parser, "@2013-12T", new ConstantExpression(P.DateTime.Parse("2013-12")));
             AssertParser.SucceedsMatch(parser, "3", new ConstantExpression(3));