diff --git a/integration_tests/comparisons_test.ucg b/integration_tests/comparisons_test.ucg
index 84ebbe8..f7456de 100644
--- a/integration_tests/comparisons_test.ucg
+++ b/integration_tests/comparisons_test.ucg
@@ -219,4 +219,19 @@ let name = "foo";
 assert {
     ok = (name) in {foo="bar"},
     desc = "(name) in {foo=\"bar\"} works",
+};
+
+assert {
+    ok = not false,
+    desc = "not false is true",
+};
+
+assert {
+    ok = not true == false,
+    desc = "not true is false",
+};
+
+assert {
+    ok = not not true,
+    desc = "double negatives are positive",
 };
\ No newline at end of file
diff --git a/src/ast/mod.rs b/src/ast/mod.rs
index a713832..bc9819f 100644
--- a/src/ast/mod.rs
+++ b/src/ast/mod.rs
@@ -601,11 +601,18 @@ pub struct FailDef {
     pub message: Box<Expression>,
 }
 
+#[derive(Debug, PartialEq, Clone)]
+pub struct NotDef {
+    pub pos: Position,
+    pub expr: Box<Expression>,
+}
+
 /// Encodes a ucg expression. Expressions compute a value from.
 #[derive(Debug, PartialEq, Clone)]
 pub enum Expression {
     // Base Expression
     Simple(Value),
+    Not(NotDef),
 
     // Binary expressions
     Binary(BinaryOpDef),
@@ -646,6 +653,7 @@ impl Expression {
             &Expression::Include(ref def) => &def.pos,
             &Expression::Import(ref def) => &def.pos,
             &Expression::Fail(ref def) => &def.pos,
+            &Expression::Not(ref def) => &def.pos,
         }
     }
 }
@@ -695,6 +703,9 @@ impl fmt::Display for Expression {
             &Expression::Fail(_) => {
                 write!(w, "<Fail>")?;
             }
+            &Expression::Not(ref def) => {
+                write!(w, "!{}", def.expr)?;
+            }
         }
         Ok(())
     }
diff --git a/src/ast/walk.rs b/src/ast/walk.rs
index d59ef66..5dce074 100644
--- a/src/ast/walk.rs
+++ b/src/ast/walk.rs
@@ -114,6 +114,9 @@ impl<'a> AstWalker<'a> {
             Expression::Import(_) | Expression::Include(_) | Expression::Fail(_) => {
                 //noop
             }
+            Expression::Not(ref mut def) => {
+                self.walk_expression(def.expr.as_mut());
+            }
         }
     }
 
diff --git a/src/build/mod.rs b/src/build/mod.rs
index ae79ed4..7f5a837 100644
--- a/src/build/mod.rs
+++ b/src/build/mod.rs
@@ -1786,7 +1786,7 @@ impl<'a> FileBuilder<'a> {
                 } else {
                     Err(Box::new(error::BuildError::new(
                         format!(
-                            "Expected string form message but got {}",
+                            "Expected string for message but got {}",
                             def.message.as_ref()
                         ),
                         error::ErrorType::TypeFail,
@@ -1794,6 +1794,21 @@ impl<'a> FileBuilder<'a> {
                     )))
                 };
             }
+            &Expression::Not(ref def) => {
+                let val = self.eval_expr(&def.expr, scope)?;
+                return if let Val::Boolean(b) = val.as_ref() {
+                    Ok(Rc::new(Val::Boolean(!b)))
+                } else {
+                    Err(Box::new(error::BuildError::new(
+                        format!(
+                            "Expected boolean for expression but got {}",
+                            def.expr.as_ref()
+                        ),
+                        error::ErrorType::TypeFail,
+                        def.expr.pos().clone(),
+                    )))
+                };
+            }
         }
     }
 }
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 08bc57d..e2bd776 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -667,6 +667,19 @@ make_fn!(
     )
 );
 
+make_fn!(
+    not_expression<SliceIter<Token>, Expression>,
+    do_each!(
+        pos => pos,
+        _ => word!("not"),
+        expr => must!(wrap_err!(expression, "Expected failure message")),
+        (Expression::Not(NotDef{
+            pos: pos,
+            expr: Box::new(expr),
+        }))
+    )
+);
+
 fn unprefixed_expression(input: SliceIter<Token>) -> ParseResult<Expression> {
     let _input = input.clone();
     either!(
@@ -685,6 +698,7 @@ make_fn!(
         trace_parse!(func_op_expression),
         trace_parse!(macro_expression),
         trace_parse!(import_expression),
+        trace_parse!(not_expression),
         trace_parse!(fail_expression),
         trace_parse!(module_expression),
         trace_parse!(select_expression),
diff --git a/src/tokenizer/mod.rs b/src/tokenizer/mod.rs
index 7ea5329..8c6ae22 100644
--- a/src/tokenizer/mod.rs
+++ b/src/tokenizer/mod.rs
@@ -278,6 +278,10 @@ make_fn!(istok<OffsetStrIter, Token>,
        do_text_token_tok!(TokenType::BAREWORD, "is", WS)
 );
 
+make_fn!(nottok<OffsetStrIter, Token>,
+       do_text_token_tok!(TokenType::BAREWORD, "not", WS)
+);
+
 make_fn!(failtok<OffsetStrIter, Token>,
        do_text_token_tok!(TokenType::BAREWORD, "fail", WS)
 );
@@ -417,6 +421,7 @@ fn token<'a>(input: OffsetStrIter<'a>) -> Result<OffsetStrIter<'a>, Token> {
         booleantok,
         intok,
         istok,
+        nottok,
         lettok,
         outtok,
         selecttok,
diff --git a/std/testing.ucg b/std/testing.ucg
index 7c6208b..32d9000 100644
--- a/std/testing.ucg
+++ b/std/testing.ucg
@@ -30,7 +30,7 @@ let asserts = module{
         // descriptive message to use in output.
         desc=mod.default_description,
     } => {
-        let ok = mod.test != true;
+        let ok = not mod.test;
 
         let desc = "@" % (mod.desc);