diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/ExpressionNode.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/ExpressionNode.java
index c1850437c..1571c7407 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/ExpressionNode.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/ExpressionNode.java
@@ -51,7 +51,7 @@ public class ExpressionNode {
     
     Empty, Not, Exists, SubsetOf, SupersetOf, IsDistinct, Distinct, Count, Where, Select, All, Repeat, Aggregate, Item /*implicit from name[]*/, As, Is, Single,
     First, Last, Tail, Skip, Take, Union, Combine, Intersect, Exclude, Iif, Upper, Lower, ToChars, IndexOf, Substring, StartsWith, EndsWith, Matches, MatchesFull, ReplaceMatches, Contains, Replace, Length,  
-    Children, Descendants, MemberOf, Trace, Check, Today, Now, Resolve, Extension, AllFalse, AnyFalse, AllTrue, AnyTrue,
+    Children, Descendants, MemberOf, Trace, DefineVariable, Check, Today, Now, Resolve, Extension, AllFalse, AnyFalse, AllTrue, AnyTrue,
     HasValue, OfType, Type, ConvertsToBoolean, ConvertsToInteger, ConvertsToString, ConvertsToDecimal, ConvertsToQuantity, ConvertsToDateTime, ConvertsToDate, ConvertsToTime, ToBoolean, ToInteger, ToString, ToDecimal, ToQuantity, ToDateTime, ToTime, ConformsTo,
     Round, Sqrt, Abs, Ceiling, Exp, Floor, Ln, Log, Power, Truncate,
     
@@ -106,6 +106,7 @@ public class ExpressionNode {
       if (name.equals("descendants")) return Function.Descendants;
       if (name.equals("memberOf")) return Function.MemberOf;
       if (name.equals("trace")) return Function.Trace;
+      if (name.equals("defineVariable")) return Function.DefineVariable;
       if (name.equals("check")) return Function.Check;
       if (name.equals("today")) return Function.Today;
       if (name.equals("now")) return Function.Now;
@@ -211,6 +212,7 @@ public class ExpressionNode {
       case Descendants : return "descendants";
       case MemberOf : return "memberOf";
       case Trace : return "trace";
+      case DefineVariable : return "defineVariable";
       case Check : return "check";
       case Today : return "today";
       case Now : return "now";
diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/FHIRPathEngine.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/FHIRPathEngine.java
index e176a84ae..28506b2d3 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/FHIRPathEngine.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/fhirpath/FHIRPathEngine.java
@@ -193,6 +193,7 @@ public class FHIRPathEngine {
     /**
      * A constant reference - e.g. a reference to a name that must be resolved in context.
      * The % will be removed from the constant name before this is invoked.
+     * Variables created with defineVariable will not be processed by resolveConstant (or resolveConstantType)
      * 
      * This will also be called if the host invokes the FluentPath engine with a context of null
      *  
@@ -1010,6 +1011,7 @@ public class FHIRPathEngine {
     private List<Base> total;
     private Map<String, Base> aliases;
     private int index;
+    private Map<String, List<Base>> definedVariables;
 
     public ExecutionContext(Object appInfo, Base resource, Base rootResource, Base context, Map<String, Base> aliases, Base thisItem) {
       this.appInfo = appInfo;
@@ -1058,6 +1060,28 @@ public class FHIRPathEngine {
       index = i;
       return this;
     }
+
+    public boolean hasDefinedVariable(String name) {
+      return definedVariables != null && definedVariables.containsKey(name);
+    }
+    public List<Base> getDefinedVariable(String name) {
+      return definedVariables == null ? makeNull() : definedVariables.get(name);
+    }
+    public void setDefinedVariable(String name, List<Base> value) {
+      if (isSystemVariable(name))
+        throw new PathEngineException(worker.formatMessage(I18nConstants.FHIRPATH_REDEFINE_VARIABLE, name), I18nConstants.FHIRPATH_REDEFINE_VARIABLE);
+
+      if (definedVariables == null) {
+        definedVariables = new HashMap<String, List<Base>>();
+      } else {
+        if (definedVariables.containsKey(name)) {
+          // Can't do this, so throw an error
+          throw new PathEngineException(worker.formatMessage(I18nConstants.FHIRPATH_REDEFINE_VARIABLE, name), I18nConstants.FHIRPATH_REDEFINE_VARIABLE);
+        }
+      }
+
+      definedVariables.put(name, value);
+    }
   }
 
   private static class ExecutionTypeContext {
@@ -1066,7 +1090,7 @@ public class FHIRPathEngine {
     private TypeDetails context;
     private TypeDetails thisItem;
     private TypeDetails total;
-
+    private Map<String, TypeDetails> definedVariables;
 
     public ExecutionTypeContext(Object appInfo, String resource, TypeDetails context, TypeDetails thisItem) {
       super();
@@ -1083,7 +1107,27 @@ public class FHIRPathEngine {
       return thisItem;
     }
 
+    public boolean hasDefinedVariable(String name) {
+      return definedVariables != null && definedVariables.containsKey(name);
+    }
+    public TypeDetails getDefinedVariable(String name) {
+      return definedVariables == null ? null : definedVariables.get(name);
+    }
+    public void setDefinedVariable(String name, TypeDetails value) {
+      if (isSystemVariable(name))
+        throw new PathEngineException("Redefine of variable "+name, I18nConstants.FHIRPATH_REDEFINE_VARIABLE);
 
+      if (definedVariables == null) {
+        definedVariables = new HashMap<String, TypeDetails>();
+      } else {
+        if (definedVariables.containsKey(name)) {
+          // Can't do this, so throw an error
+          throw new PathEngineException("Redefine of variable "+name, I18nConstants.FHIRPATH_REDEFINE_VARIABLE);
+        }
+      }
+
+      definedVariables.put(name, value);
+    }
   }
 
   private ExpressionNode parseExpression(FHIRLexer lexer, boolean proximal) throws FHIRLexerException {
@@ -1423,6 +1467,7 @@ public class FHIRPathEngine {
     case Descendants: return checkParamCount(lexer, location, exp, 0);
     case MemberOf: return checkParamCount(lexer, location, exp, 1);
     case Trace: return checkParamCount(lexer, location, exp, 1, 2);
+    case DefineVariable: return checkParamCount(lexer, location, exp, 1, 2);
     case Check: return checkParamCount(lexer, location, exp, 2);
     case Today: return checkParamCount(lexer, location, exp, 0);
     case Now: return checkParamCount(lexer, location, exp, 0);
@@ -1480,8 +1525,9 @@ public class FHIRPathEngine {
     return false;
   }
 
-  private List<Base> execute(ExecutionContext context, List<Base> focus, ExpressionNode exp, boolean atEntry) throws FHIRException {
+  private List<Base> execute(ExecutionContext inContext, List<Base> focus, ExpressionNode exp, boolean atEntry) throws FHIRException {
     //    System.out.println("Evaluate {'"+exp.toString()+"'} on "+focus.toString());
+    ExecutionContext context = contextForParameter(inContext);
     List<Base> work = new ArrayList<Base>();
     switch (exp.getKind()) {
     case Unary:
@@ -1506,7 +1552,7 @@ public class FHIRPathEngine {
       }
       break;
     case Function:
-      List<Base> work2 = evaluateFunction(context, focus, exp);
+      List<Base> work2 = evaluateFunction("aliasAs".equals(exp.getName()) ? inContext : context, focus, exp);
       work.addAll(work2);
       break;
     case Constant:
@@ -1525,6 +1571,7 @@ public class FHIRPathEngine {
       ExpressionNode next = exp.getOpNext();
       ExpressionNode last = exp;
       while (next != null) {
+        context = contextForParameter(inContext);
         List<Base> work2 = preOperate(work, last.getOperation(), exp);
         if (work2 != null) {
           work = work2;
@@ -1588,8 +1635,8 @@ public class FHIRPathEngine {
     return new TypeDetails(CollectionStatus.SINGLETON, exp.getName());
   }
 
-  private TypeDetails executeType(ExecutionTypeContext context, TypeDetails focus, ExpressionNode exp, Set<ElementDefinition> elementDependencies, boolean atEntry, boolean canBeNone, ExpressionNode container) throws PathEngineException, DefinitionException {
-    
+  private TypeDetails executeType(ExecutionTypeContext inContext, TypeDetails focus, ExpressionNode exp, Set<ElementDefinition> elementDependencies, boolean atEntry, boolean canBeNone, ExpressionNode container) throws PathEngineException, DefinitionException {
+    ExecutionTypeContext context = contextForParameter(inContext);
     TypeDetails result = new TypeDetails(null);
     switch (exp.getKind()) {
     case Name:
@@ -1639,6 +1686,7 @@ public class FHIRPathEngine {
       ExpressionNode next = exp.getOpNext();
       ExpressionNode last = exp;
       while (next != null) {
+        context = contextForParameter(inContext);
         TypeDetails work;
         if (last.getOperation() == Operation.Is || last.getOperation() == Operation.As) {
           work = executeTypeName(context, focus, next, atEntry);
@@ -1676,6 +1724,10 @@ public class FHIRPathEngine {
     }
     FHIRConstant c = (FHIRConstant) constant;
     if (c.getValue().startsWith("%")) {
+      String varName = c.getValue().substring(1);
+      if (context.hasDefinedVariable(varName)) {
+        return context.getDefinedVariable(varName);
+      }
       return resolveConstant(context, c.getValue(), beforeContext, expr, true);
     } else if (c.getValue().startsWith("@")) {
       return new ArrayList<Base>(Arrays.asList(processDateConstant(context.appInfo, c.getValue().substring(1), expr)));
@@ -1746,6 +1798,21 @@ public class FHIRPathEngine {
     }
   }
 
+  static boolean isSystemVariable(String name){
+    if (name.equals("sct"))
+      return true;
+    if (name.equals("loinc"))
+      return true;
+    if (name.equals("ucum"))
+      return true;
+    if (name.equals("resource"))
+      return true;
+    if (name.equals("rootResource"))
+      return true;
+    if (name.equals("context"))
+      return true;
+    return false;
+  }
 
   private List<Base> resolveConstant(ExecutionContext context, String s, boolean beforeContext, ExpressionNode expr, boolean explicitConstant) throws PathEngineException {
     if (s.equals("%sct")) {
@@ -3151,6 +3218,10 @@ public class FHIRPathEngine {
     } else if (hostServices == null) {
       throw makeException(expr, I18nConstants.FHIRPATH_UNKNOWN_CONSTANT, s);
     } else {
+      String varName = s.substring(1);
+      if (context.hasDefinedVariable(varName)) {
+        return context.getDefinedVariable(varName);
+      }
       TypeDetails v = hostServices.resolveConstantType(this, context.appInfo, s, explicitConstant);
       if (v == null) {
         throw makeException(expr, I18nConstants.FHIRPATH_UNKNOWN_CONSTANT, s); 
@@ -3503,6 +3574,25 @@ public class FHIRPathEngine {
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return focus; 
     }
+    case DefineVariable : {
+      checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.UNORDERED, TypeDetails.FP_String)); 
+      // set the type of the variable
+      // Actually evaluate the value of the first parameter (to get the name of the variable if possible)
+      // and if have that, set it into the context
+      ExpressionNode p = exp.getParameters().get(0);
+      if (p.getKind() == Kind.Constant && p.getConstant() != null) {
+        String varName = exp.getParameters().get(0).getConstant().primitiveValue();
+        if (varName != null) {
+          if (paramTypes.size() > 1)
+            context.setDefinedVariable(varName, paramTypes.get(1));
+          else
+            context.setDefinedVariable(varName, focus);
+        }
+      } else {
+        // this variable is not a constant, so we can't analyze what name it could have
+      }
+      return focus; 
+    }
     case Check : {
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return focus; 
@@ -3734,7 +3824,7 @@ public class FHIRPathEngine {
     case 0:
       return exp.getFunction() == Function.Where || exp.getFunction() == Function.Exists || exp.getFunction() == Function.All || exp.getFunction() == Function.Select || exp.getFunction() == Function.Repeat || exp.getFunction() == Function.Aggregate;
     case 1:
-      return exp.getFunction() == Function.Trace;
+      return exp.getFunction() == Function.Trace || exp.getFunction() == Function.DefineVariable;
     default: 
       return false;
     }
@@ -3887,6 +3977,7 @@ public class FHIRPathEngine {
     case Descendants : return funcDescendants(context, focus, exp);
     case MemberOf : return funcMemberOf(context, focus, exp);
     case Trace : return funcTrace(context, focus, exp);
+    case DefineVariable : return funcDefineVariable(context, focus, exp);
     case Check : return funcCheck(context, focus, exp);
     case Today : return funcToday(context, focus, exp);
     case Now : return funcNow(context, focus, exp);
@@ -4652,13 +4743,50 @@ public class FHIRPathEngine {
 
 
   private ExecutionContext changeThis(ExecutionContext context, Base newThis) {
-    return new ExecutionContext(context.appInfo, context.focusResource, context.rootResource, context.context, context.aliases, newThis);
+    ExecutionContext newContext = new ExecutionContext(context.appInfo, context.focusResource, context.rootResource, context.context, context.aliases, newThis);
+    // append all of the defined variables from the context into the new context
+    if (context.definedVariables != null) {
+      for (String s : context.definedVariables.keySet()) {
+        newContext.setDefinedVariable(s, context.definedVariables.get(s));
+      }
+    }
+    return newContext;
+  }
+
+  private ExecutionContext contextForParameter(ExecutionContext context) {
+    ExecutionContext newContext = new ExecutionContext(context.appInfo, context.focusResource, context.rootResource, context.context, context.aliases, context.thisItem);
+    newContext.total = context.total;
+    newContext.index = context.index;
+    // append all of the defined variables from the context into the new context
+    if (context.definedVariables != null) {
+      for (String s : context.definedVariables.keySet()) {
+        newContext.setDefinedVariable(s, context.definedVariables.get(s));
+      }
+    }
+    return newContext;
   }
 
   private ExecutionTypeContext changeThis(ExecutionTypeContext context, TypeDetails newThis) {
-    return new ExecutionTypeContext(context.appInfo, context.resource, context.context, newThis);
+    ExecutionTypeContext newContext = new ExecutionTypeContext(context.appInfo, context.resource, context.context, newThis);
+    // append all of the defined variables from the context into the new context
+    if (context.definedVariables != null) {
+      for (String s : context.definedVariables.keySet()) {
+        newContext.setDefinedVariable(s, context.definedVariables.get(s));
+      }
+    }
+    return newContext;
   }
 
+  private ExecutionTypeContext contextForParameter(ExecutionTypeContext context) {
+    ExecutionTypeContext newContext = new ExecutionTypeContext(context.appInfo, context.resource, context.context, context.thisItem);
+    // append all of the defined variables from the context into the new context
+    if (context.definedVariables != null) {
+      for (String s : context.definedVariables.keySet()) {
+        newContext.setDefinedVariable(s, context.definedVariables.get(s));
+      }
+    }
+    return newContext;
+  }
 
   private List<Base> funcNow(ExecutionContext context, List<Base> focus, ExpressionNode exp) {
     List<Base> result = new ArrayList<Base>();
@@ -5496,6 +5624,20 @@ public class FHIRPathEngine {
     return focus;
   }
 
+  private List<Base> funcDefineVariable(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
+    List<Base> nl = execute(context, focus, exp.getParameters().get(0), true);
+    String name = nl.get(0).primitiveValue();
+    List<Base> value;
+    if (exp.getParameters().size() == 2) {
+      value = execute(context, focus, exp.getParameters().get(1), true);
+    } else { 
+      value = focus;
+    }
+    // stash the variable into the context
+    context.setDefinedVariable(name, value);
+    return focus;
+  }
+
   private List<Base> funcCheck(ExecutionContext context, List<Base> focus, ExpressionNode expr) throws FHIRException {
     List<Base> n1 = execute(context, focus, expr.getParameters().get(0), true);
     if (!convertToBoolean(n1)) {
diff --git a/org.hl7.fhir.r5/src/test/java/org/hl7/fhir/r5/test/FHIRPathTests.java b/org.hl7.fhir.r5/src/test/java/org/hl7/fhir/r5/test/FHIRPathTests.java
index aefedf7f6..0cdf20a34 100644
--- a/org.hl7.fhir.r5/src/test/java/org/hl7/fhir/r5/test/FHIRPathTests.java
+++ b/org.hl7.fhir.r5/src/test/java/org/hl7/fhir/r5/test/FHIRPathTests.java
@@ -180,6 +180,9 @@ public class FHIRPathTests {
       fail = TestResultType.EXECUTION;      
     };
     fp.setAllowPolymorphicNames("lenient/polymorphics".equals(test.getAttribute("mode")));
+    boolean skipStaticCheck = false;
+    if ("true".equals(test.getAttribute("skipStaticCheck")))
+      skipStaticCheck = true;
     Base res = null;
 
     List<Base> outcome = new ArrayList<Base>();
@@ -210,17 +213,19 @@ public class FHIRPathTests {
         }        
       }
       
-      try {
-        if (Utilities.noString(input)) {
-          fp.check(null, null, node);
-        } else {
-          fp.check(res, res.fhirType(), res.fhirType(), node);
+      if (!skipStaticCheck) {
+        try {
+          if (Utilities.noString(input)) {
+            fp.check(null, null, node);
+          } else {
+            fp.check(res, res.fhirType(), res.fhirType(), node);
+          }
+          Assertions.assertTrue(fail != TestResultType.SEMANTICS, String.format("Expected exception didn't occur checking %s", expression));
+        } catch (Exception e) {
+          System.out.println("Checking Error: "+e.getMessage());
+          Assertions.assertTrue(fail == TestResultType.SEMANTICS, String.format("Unexpected exception checking %s: " + e.getMessage(), expression));
+          node = null;
         }
-        Assertions.assertTrue(fail != TestResultType.SEMANTICS, String.format("Expected exception didn't occur checking %s", expression));
-      } catch (Exception e) {
-        System.out.println("Checking Error: "+e.getMessage());
-        Assertions.assertTrue(fail == TestResultType.SEMANTICS, String.format("Unexpected exception checking %s: " + e.getMessage(), expression));
-        node = null;
       }
     }
     
diff --git a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
index 7968cb2e7..9e481a680 100644
--- a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
+++ b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
@@ -1102,6 +1102,7 @@ public class I18nConstants {
   public static final String SD_ELEMENT_FIXED_WRONG_TYPE = "SD_ELEMENT_FIXED_WRONG_TYPE";
   public static final String SD_ELEMENT_REASON_DERIVED = "SD_ELEMENT_REASON_DERIVED";
   public static final String SD_ELEMENT_PATTERN_WRONG_TYPE = "SD_ELEMENT_PATTERN_WRONG_TYPE";
+  public static final String FHIRPATH_REDEFINE_VARIABLE = "FHIRPATH_REDEFINE_VARIABLE";
 }
 
 
diff --git a/org.hl7.fhir.utilities/src/main/resources/Messages.properties b/org.hl7.fhir.utilities/src/main/resources/Messages.properties
index 7002b84bd..5dabcf717 100644
--- a/org.hl7.fhir.utilities/src/main/resources/Messages.properties
+++ b/org.hl7.fhir.utilities/src/main/resources/Messages.properties
@@ -1084,6 +1084,7 @@ TX_GENERAL_CC_ERROR_MESSAGE = No valid coding was found for the value set ''{0}'
 Validation_VAL_Profile_Minimum_SLICE_one = Slice ''{3}'': a matching slice is required, but not found (from {1}). Note that other slices are allowed in addition to this required slice
 Validation_VAL_Profile_Minimum_SLICE_other = Slice ''{3}'': minimum required = {0}, but only found {7} (from {1})
 FHIRPATH_UNKNOWN_EXTENSION = Reference to an unknown extension - double check that the URL ''{0}'' is correct
+FHIRPATH_REDEFINE_VARIABLE = The variable ''{0}'' cannot be redefined
 Type_Specific_Checks_DT_XHTML_Resolve = Hyperlink ''{0}'' at ''{1}'' for ''{2}''' does not resolve
 Type_Specific_Checks_DT_XHTML_Resolve_Img = Image source ''{0}'' at ''{1}'' does not resolve
 TYPE_SPECIFIC_CHECKS_DT_XHTML_MULTIPLE_MATCHES = Hyperlink ''{0}'' at ''{1}'' for ''{2}'' resolves to multiple targets ({3})