diff --git a/DDMathParser/DDMathEvaluator.m b/DDMathParser/DDMathEvaluator.m
index 5dbc6d3..1644a29 100644
--- a/DDMathParser/DDMathEvaluator.m
+++ b/DDMathParser/DDMathEvaluator.m
@@ -271,6 +271,7 @@ + (NSSet *) _standardFunctions {
                              //logical functions
                              @"l_and",
                              @"l_or",
+                             @"l_not",
                              nil];
     });
 	return standardFunctions;
diff --git a/DDMathParser/DDMathStringTokenizer.m b/DDMathParser/DDMathStringTokenizer.m
index 9e49b88..5a4aae3 100644
--- a/DDMathParser/DDMathStringTokenizer.m
+++ b/DDMathParser/DDMathStringTokenizer.m
@@ -92,12 +92,12 @@ - (id)initWithString:(NSString *)expressionString error:(NSError **)error {
                 return nil;
             }
         }
+        
+        [self _processToken:nil withError:error];
 		
         if (error && *error) {
             DD_RELEASE(self);
             self = nil;
-        } else {
-            [self _processToken:nil withError:nil];
         }
     }
     
@@ -119,10 +119,7 @@ - (void)dealloc {
 
 - (BOOL)_processToken:(DDMathStringToken *)token withError:(NSError **)error {
     //figure out if "-" and "+" are unary or binary
-    BOOL shouldContinue = [self _processUnknownOperatorToken:token withError:error];
-    if (!shouldContinue) {
-        return NO;
-    }
+    (void)[self _processUnknownOperatorToken:token withError:error];
     
     if ([token operatorType] == DDOperatorUnaryPlus) {
         // the unary + operator is a no-op operator.  It does nothing, so we'll throw it out
@@ -140,8 +137,8 @@ - (BOOL)_processToken:(DDMathStringToken *)token withError:(NSError **)error {
 }
 
 - (BOOL)_processUnknownOperatorToken:(DDMathStringToken *)token withError:(NSError **)error {
+    DDMathStringToken *previousToken = [_tokens lastObject];   
     if ([token tokenType] == DDTokenTypeOperator && [token operatorType] == DDOperatorInvalid) {
-        DDMathStringToken *previousToken = [_tokens lastObject];   
         DDOperator resolvedOperator = DDOperatorInvalid;
         
         BOOL shouldBeUnary = NO;
@@ -168,6 +165,14 @@ - (BOOL)_processUnknownOperatorToken:(DDMathStringToken *)token withError:(NSErr
             }
         }
         
+        if ([[token token] isEqual:@"!"]) {
+            if (previousToken == nil) {
+                resolvedOperator = DDOperatorLogicalNot;
+            } else if ([previousToken tokenType] == DDTokenTypeOperator && [previousToken operatorType] != DDOperatorParenthesisClose) {
+                resolvedOperator = DDOperatorLogicalNot;
+            }
+        }
+        
         [token resolveToOperator:resolvedOperator];
         
         if ([token operatorType] == DDOperatorInvalid) {
@@ -177,6 +182,14 @@ - (BOOL)_processUnknownOperatorToken:(DDMathStringToken *)token withError:(NSErr
             return NO;
         }
     }
+    
+    if (token == nil && [[previousToken token] isEqual:@"!"]) {
+        [previousToken resolveToOperator:DDOperatorFactorial];
+        if (error != nil) {
+            *error = nil;
+        }
+    }
+    
     return YES;
 }
 
diff --git a/DDMathParser/DDParser.m b/DDMathParser/DDParser.m
index 9bd634b..468dfd7 100644
--- a/DDMathParser/DDParser.m
+++ b/DDMathParser/DDParser.m
@@ -145,6 +145,8 @@ - (DDOperatorAssociativity) associativityForOperator:(DDOperator)operatorType {
             
         // factorial is always left associative
         case DDOperatorFactorial: return DDOperatorAssociativityLeft;
+        // logical not is always right associative
+        case DDOperatorLogicalNot: return DDOperatorAssociativityRight;
             
 		default: return DDOperatorAssociativityLeft;
 	}
diff --git a/DDMathParser/DDParserTypes.h b/DDMathParser/DDParserTypes.h
index cd1d9f6..050fc1d 100644
--- a/DDMathParser/DDParserTypes.h
+++ b/DDMathParser/DDParserTypes.h
@@ -25,6 +25,7 @@ typedef enum {
     
     DDOperatorLogicalOr,
     DDOperatorLogicalAnd,
+    DDOperatorLogicalNot,
 	
 	DDOperatorBitwiseOr,
 	DDOperatorBitwiseXor,
diff --git a/DDMathParser/NSString+DDMathParsing.m b/DDMathParser/NSString+DDMathParsing.m
index 7ee3018..a6d0538 100644
--- a/DDMathParser/NSString+DDMathParsing.m
+++ b/DDMathParser/NSString+DDMathParsing.m
@@ -18,9 +18,8 @@ - (NSNumber *) numberByEvaluatingString {
 - (NSNumber *) numberByEvaluatingStringWithSubstitutions:(NSDictionary *)substitutions {
 	NSError *error = nil;
 	NSNumber *returnValue = [self numberByEvaluatingStringWithSubstitutions:substitutions error:&error];
-	if (error != nil) {
+	if (returnValue == nil) {
 		NSLog(@"error: %@", error);
-		return nil;
 	}
 	return returnValue;
 }
diff --git a/DDMathParser/_DDFunctionUtilities.h b/DDMathParser/_DDFunctionUtilities.h
index 4f932ac..d8d086a 100644
--- a/DDMathParser/_DDFunctionUtilities.h
+++ b/DDMathParser/_DDFunctionUtilities.h
@@ -98,5 +98,6 @@
 
 + (DDMathFunction) l_andFunction;
 + (DDMathFunction) l_orFunction;
++ (DDMathFunction) l_notFunction;
 
 @end
diff --git a/DDMathParser/_DDFunctionUtilities.m b/DDMathParser/_DDFunctionUtilities.m
index a020e32..8927d9b 100644
--- a/DDMathParser/_DDFunctionUtilities.m
+++ b/DDMathParser/_DDFunctionUtilities.m
@@ -1097,4 +1097,16 @@ + (DDMathFunction) l_orFunction {
 	return DD_AUTORELEASE([function copy]);
 }
 
++ (DDMathFunction) l_notFunction {
+	DDMathFunction function = ^ DDExpression* (NSArray *arguments, NSDictionary *variables, DDMathEvaluator *evaluator, NSError **error) {
+#pragma unused(variables, evaluator)
+		REQUIRE_N_ARGS(1);
+		NSNumber *n = [[arguments objectAtIndex:0] evaluateWithSubstitutions:variables evaluator:evaluator error:error];
+        NSNumber *result = [NSNumber numberWithBool:![n boolValue]];
+		return [DDExpression numberExpressionWithNumber:result];
+		
+	};
+	return DD_AUTORELEASE([function copy]);
+}
+
 @end
diff --git a/DDMathParser/_DDOperatorInfo.m b/DDMathParser/_DDOperatorInfo.m
index 179e954..7d95f15 100644
--- a/DDMathParser/_DDOperatorInfo.m
+++ b/DDMathParser/_DDOperatorInfo.m
@@ -50,6 +50,9 @@ + (NSArray *)_buildOperators {
     [operators addObject:[self infoForOperator:DDOperatorLogicalAnd arity:DDOperatorArityBinary precedence:precedence token:@"&&" function:@"l_and"]];
     precedence++;
     
+    [operators addObject:[self infoForOperator:DDOperatorLogicalNot arity:DDOperatorArityUnary precedence:precedence token:@"!" function:@"l_not"]];
+    precedence++;
+    
     [operators addObject:[self infoForOperator:DDOperatorBitwiseOr arity:DDOperatorArityBinary precedence:precedence token:@"|" function:@"or"]];
     precedence++;