diff --git a/resources/koans.clj b/resources/koans.clj
index fcf1dc8..6f35a56 100644
--- a/resources/koans.clj
+++ b/resources/koans.clj
@@ -209,4 +209,14 @@
                         5
                         :hello
                         (6 :these :are)
-                        ]}]]
+                        ]}]
+ ["21_group_by" {"__" [odd?
+                       {5 ["hello" "world"] 3 ["foo" "bar"]}
+                       {1 [{:name "Bob" :id 1}
+                           {:last-name "Smith" :id 1}]
+                        2 [{:name "Mike" :id 2}]}
+                       nil
+                       {:nauty-list [{:name "Jimmy" :bad true}
+                                     {:name "Joe" :bad true}]
+                        :nice-list [{:name "Jack" :bad false}]}]}]
+]
diff --git a/src/koans/21_group_by.clj b/src/koans/21_group_by.clj
new file mode 100644
index 0000000..245a95a
--- /dev/null
+++ b/src/koans/21_group_by.clj
@@ -0,0 +1,32 @@
+(defn get-odds-and-evens [coll]
+  (let [{odds true evens false} (group-by __ coll)]
+    [odds evens]))
+
+(meditations
+
+  "To split a collection categorized by keys use the group-by function."
+  (= __ (group-by count ["hello" "world" "foo" "bar"]))
+
+
+  "Sometimes you want to filter a sequence but also keep the predicate."
+  (= (get-odds-and-evens '(1 2 3 4 5))
+    [[1 3 5] [2 4]])
+
+  "You can also group by a primary key"
+  (= __
+     (group-by :id [{:id 1 :name "Bob"}
+                   {:id 2 :name "Mike"}
+                   {:id 1 :last-name "Smith"} ]))
+
+  "but be careful when you group by non primary key"
+  (= {"Bob" [{:name "Bob" :id 1}], "Mike" [{:name "Mike" :id 2}], __ [{:last-name "Smith" :id 1}]}
+   (group-by :name [{:id 1 :name "Bob"}
+                    {:id 2 :name "Mike"}
+                    {:id 1 :last-name "Smith"}]))
+
+  "The true power of group-by is using a custom function"
+  (= __
+     (group-by #(if (:bad %) :nauty-list :nice-list)
+               [{:name "Jimmy" :bad true}
+                {:name "Jack" :bad false}
+                {:name "Joe" :bad true}])))