diff --git a/Classes/NSArray+ObjectiveSugar.h b/Classes/NSArray+ObjectiveSugar.h
index 2562eb1..aef04d2 100644
--- a/Classes/NSArray+ObjectiveSugar.h
+++ b/Classes/NSArray+ObjectiveSugar.h
@@ -152,6 +152,14 @@
  */
 - (NSArray *)reject:(BOOL (^)(id object))block;
 
+/**
+ Iterate through current array adding the element to a dictionary asking which key to use.
+ 
+ @param A block that returns the key that should be used to add the element to the dictionary
+ @return A dictionary with the elements grouped by the key
+ */
+- (NSDictionary *)groupBy:(id (^)(id object))key;
+
 /**
  Recurse through self checking for NSArrays and extract all elements into one single array
 
diff --git a/Classes/NSArray+ObjectiveSugar.m b/Classes/NSArray+ObjectiveSugar.m
index 82c28f5..d86372e 100644
--- a/Classes/NSArray+ObjectiveSugar.m
+++ b/Classes/NSArray+ObjectiveSugar.m
@@ -114,6 +114,17 @@ - (NSArray *)reject:(BOOL (^)(id object))block {
     }]];
 }
 
+- (NSDictionary *)groupBy:(id (^)(id object))key {
+    return [self reduce:[NSMutableDictionary dictionary] withBlock:^id(NSMutableDictionary *accumulator, id object) {
+        if (!accumulator[key(object)]) {
+            accumulator[key(object)] = [NSMutableArray array];
+        }
+        [accumulator[key(object)] addObject:object];
+        return accumulator;
+    }];
+}
+
+
 - (id)detect:(BOOL (^)(id object))block {
     for (id object in self) {
         if (block(object))
diff --git a/Example/ObjectiveSugarTests/NSArrayTests.m b/Example/ObjectiveSugarTests/NSArrayTests.m
index 651d474..b24ae3d 100644
--- a/Example/ObjectiveSugarTests/NSArrayTests.m
+++ b/Example/ObjectiveSugarTests/NSArrayTests.m
@@ -145,6 +145,16 @@
           return @(accumulator.intValue + numbah.intValue);
         }] should] equal:@60];
     });
+    
+    it(@"-groupBy: returns a dictionary grouped with the specified keys", ^{
+        NSDictionary *groupedByPairs = [oneToTen groupBy:^id(NSNumber *number) {
+            return @([number intValue] % 2 == 0);
+        }];
+        [[groupedByPairs[@YES] should] containObjects:@2, @4, @6, @8, @10, nil];
+        [[groupedByPairs[@YES] shouldNot] containObjects:@1, @3, @5, @7, @9, nil];
+        [[groupedByPairs[@NO] should] containObjects:@1, @3, @5, @7, @9, nil];
+        [[groupedByPairs[@NO] shouldNot] containObjects:@2, @4, @6, @8, @10, nil];
+    });
   
     it(@"-unique produces a duplicate-free array", ^{
       NSArray *arrayWithDuplicates = @[@1, @1, @"something", @"something"];
diff --git a/Example/ObjectiveSugarTests/NSStringTests.m b/Example/ObjectiveSugarTests/NSStringTests.m
index df96cd8..245c744 100644
--- a/Example/ObjectiveSugarTests/NSStringTests.m
+++ b/Example/ObjectiveSugarTests/NSStringTests.m
@@ -15,7 +15,7 @@
 
     it(@"NSStringWithFormat makes NSString -stringWithFormat", ^{
 
-        [[NSStringWithFormat(@"This is %@", @1) should] equal:@"This is 1"];
+        [[[NSString stringWithFormat:@"This is %@", @1] should] equal:@"This is 1"];
     });
 
 });
@@ -55,14 +55,14 @@
 
     context(@"lowerCamelCase", ^{
         it(@"converts snake_case to snakeCase", ^{
-            [[[@"snake_case" lowerCamelCase] should] equal:@"snakeCase"]
+            [[[@"snake_case" lowerCamelCase] should] equal:@"snakeCase"];
         });
 
         it(@"handles extraneous underscores", ^{
-            [[[@"_snake_case" lowerCamelCase] should] equal:@"snakeCase"]
-            [[[@"snake_case_" lowerCamelCase] should] equal:@"snakeCase"]
+            [[[@"_snake_case" lowerCamelCase] should] equal:@"snakeCase"];
+            [[[@"snake_case_" lowerCamelCase] should] equal:@"snakeCase"];
         });
-    }
+    });
 
     it(@"-strip strips whitespaces and newlines from both ends", ^{
         [[[@"\n  Look mo, no empties!\n \n\n  " strip] should] equal:@"Look mo, no empties!"];
diff --git a/README.md b/README.md
index f7fa44c..197b333 100644
--- a/README.md
+++ b/README.md
@@ -166,6 +166,13 @@ numbers[range];
 [numbers reverse];
 // [@6, @5, @4, @3, @2, @1]
 
+// group pair numbers
+[numbers groupBy:^id(NSNumber *number) {
+    return @([number intValue] % 2 == 0);
+}];
+// @{@NO  : @[@1, @3, @5],
+//   @YES : @[@2, @4, @6]}
+
 
 NSArray *fruits = @[ @"banana", @"mango", @"apple", @"pear" ];