diff --git a/bench/map.carp b/bench/map.carp
index 93105214..96f559a3 100644
--- a/bench/map.carp
+++ b/bench/map.carp
@@ -7,7 +7,7 @@
       (Map.put! &m &i &1))))
 
 (defn insert-collisions []
-  (let [m (Map.create-with-len 1)]
+  (let [m {}]
     (for [i 0 100]
       (Map.put! &m &1 &1))))
 
@@ -23,11 +23,6 @@
 (defn retrieve []
   (Map.get &m &10))
 
-(defn setup-big-map-collisions []
-  (do
-    (set! m (Map.create-with-len 1))
-    (setup-big-map)))
-
 (defn map-tests []
   (do
     (println "Testing single map insert:")
@@ -42,10 +37,6 @@
     (setup-big-map)
     (println "Testing map retrieval:")
     (bench retrieve)
-    (println "")
-    (setup-big-map-collisions)
-    (println "Testing map retrieval with maximum collisions:")
-    (bench retrieve)
     (println "")))
 
 
@@ -55,7 +46,7 @@
       (Set.put! &m &i))))
 
 (defn insert-set-collisions []
-  (let [m (Set.create-with-len 1)]
+  (let [m (Set.create)]
     (for [i 0 100]
       (Set.put! &m &i))))
 
@@ -71,11 +62,6 @@
 (defn contains-set []
   (Set.contains? &s &10))
 
-(defn setup-big-set-collisions []
-  (do
-    (set! s (Set.create-with-len 1))
-    (setup-big-set)))
-
 (defn set-tests []
   (do
     (println "Testing single set insert:")
@@ -90,10 +76,6 @@
     (setup-big-set)
     (println "Testing set contains:")
     (bench contains-set)
-    (println "")
-    (setup-big-set-collisions)
-    (println "Testing set contains with maximum collisions:")
-    (bench contains-set)
     (println "")))
 
 (defn main []
diff --git a/core/Map.carp b/core/Map.carp
index 7f879e57..6bc8058e 100644
--- a/core/Map.carp
+++ b/core/Map.carp
@@ -175,34 +175,72 @@
       b))
 )
 
-(deftype (Map a b) [n-buckets Int buckets (Array (Bucket a b))])
+(deftype (Map a b) [len Int n-buckets Int buckets (Array (Bucket a b))])
 
+(doc Map
+  "is a hashmap datatype, i.e. a key-value structure. It allows you to put in "
+  "pairs of keys and values and look them up by key."
+  ""
+  "Implementation notes: it is a dense double-array-backed structure that"
+  "grows and shrinks based on load.")
 (defmodule Map
   (hidden dflt-len)
   (def dflt-len 256)
 
+  (doc min-load "The load a map needs to reach in order to shrink.")
+  (def min-load 20)
+  (doc max-load "The load a map needs to reach in order to grow.")
+  (def max-load 80)
+
   (doc create "Create an empty map.")
   (defn create []
-    (init dflt-len (Array.repeat dflt-len &Bucket.empty)))
+    (init 0 dflt-len (Array.repeat dflt-len &Bucket.empty)))
 
-  (doc create-with-len "Create an empty map with a given number of buckets. High numbers reduce the possibility of hash collisions while increasing the memory footprint.")
-  (defn create-with-len [len]
-    (init len (Array.repeat len &Bucket.empty)))
-
-  (doc put "Put a a value v into map m, using the key k.")
-  (defn put [m k v]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets &m))]
-      (update-buckets m &(fn [b]
-                           (let [n (Array.unsafe-nth &b idx)]
-                             (Array.aset b idx (Bucket.put @n k v)))))))
-
-  (doc put! "Put a a value v into map m, using the key k, in place.")
-  (defn put! [m k v]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets m))
-          b (buckets m)
-          n (Array.unsafe-nth b idx)]
+  (private put-!)
+  (hidden put-!)
+  (defn put-! [m k v]
+    (let-do [idx (Int.positive-mod (hash k) @(n-buckets m))
+             b (buckets m)
+             n (Array.unsafe-nth b idx)]
+      (set-len! m (Int.inc @(len m)))
       (Bucket.put! n k v)))
 
+  (doc resize "Resize a map `m` to size `s`.")
+  (defn resize [m s]
+    (let-do [n (init 0 s (Array.repeat s &Bucket.empty))]
+      (for [i 0 @(n-buckets &m)]
+        (let [bucket (Array.unsafe-nth (buckets &m) i)
+              len (Array.length (Bucket.entries bucket))
+              entries (Bucket.entries bucket)]
+          (for [j 0 len]
+            (let [e (Array.unsafe-nth entries j)]
+              (put-! &n (Pair.a e) (Pair.b e))))))
+        n))
+
+  (doc grow "Grow a map `m`. Should usually be handled automatically.")
+  (defn grow [m] (resize m (* @(n-buckets &m) 2)))
+  (doc shrink "Shrink a map `m`. Should usually be handled automatically.")
+  (defn shrink [m]
+    (let [new-size (/ @(n-buckets &m) 2)]
+      (if (< new-size dflt-len)
+        m
+        (resize m new-size))))
+
+  (doc put "Put a value v into map m, using the key k.")
+  (defn put [m k v]
+    (if (> (/ (* @(len &m) 100) @(n-buckets &m)) min-load)
+      (put (grow m) k v)
+      (let [idx (Int.positive-mod (hash k) @(n-buckets &m))]
+        (update-len
+          (update-buckets m &(fn [b]
+                               (let [n (Array.unsafe-nth &b idx)]
+                                 (Array.aset b idx (Bucket.put @n k v)))))
+           &Int.inc))))
+
+  (doc put! "Put a value v into map m, using the key k, in place.")
+  (defn put! [m k v]
+    (put-! m k v))
+
   (doc get-with-default "Get the value for the key k from map m. If it isn’t found, the default is returned.")
   (defn get-with-default [m k default-value]
     (let [idx (Int.positive-mod (hash k) @(n-buckets m))]
@@ -240,14 +278,11 @@
 
   (doc length "Get the length of the map m.")
   (defn length [m]
-    (let-do [c 0]
-      (for [i 0 @(n-buckets m)]
-        (set! c (+ c (Array.length (Bucket.entries (Array.unsafe-nth (buckets m) i))))))
-      c))
+    @(len m))
 
   (doc empty? "Check whether the map m is empty.")
   (defn empty? [m]
-    (= (length m) 0))
+    (= @(len m) 0))
 
   (doc contains? "Check whether the map m contains the key k.")
   (defn contains? [m k]
@@ -256,10 +291,14 @@
 
   (doc remove "Remove the value under the key k from the map m.")
   (defn remove [m k]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets &m))]
-      (update-buckets m &(fn [b]
-                           (let [n (Array.unsafe-nth &b idx)]
-                             (Array.aset b idx (Bucket.shrink @n k)))))))
+    (if (> (/ (* @(len &m) 100) @(n-buckets &m)) min-load)
+      (remove (shrink m) k)
+      (let [idx (Int.positive-mod (hash k) @(n-buckets &m))]
+        (update-len
+          (update-buckets m &(fn [b]
+                               (let [n (Array.unsafe-nth &b idx)]
+                                 (Array.aset b idx (Bucket.shrink @n k)))))
+          &Int.dec))))
 
   (doc all? "Do all key-value pairs pass the given predicate (of two arguments)?")
   (defn all? [pred m]
@@ -334,7 +373,7 @@
         (let [e (Array.unsafe-nth a i)
               k (Pair.a e)
               v (Pair.b e)]
-          (set! m (put m k v))))
+          (put! &m k v)))
       m))
 
   (doc to-array "Convert Map to Array of Pairs")
@@ -392,62 +431,105 @@
       @b))
 )
 
-(deftype (Set a) [n-buckets Int buckets (Array (SetBucket a))])
+(deftype (Set a) [len Int n-buckets Int buckets (Array (SetBucket a))])
 
+(doc Set
+  "is a hashset datatype, i.e. a unique list structure. It allows you to put "
+  "in values and guarantees uniqueness."
+  ""
+  "Implementation notes: it is a dense double-array-backed structure that"
+  "grows and shrinks based on load, similar to `Map`.")
 (defmodule Set
   (hidden dflt-len)
   (def dflt-len 256)
 
+  (doc min-load "The load a set needs to reach in order to shrink.")
+  (def min-load 20)
+  (doc max-load "The load a set needs to reach in order to grow.")
+  (def max-load 80)
+
   (doc create "Create an empty set.")
   (defn create []
-    (init dflt-len (Array.repeat dflt-len &SetBucket.empty)))
+    (init 0 dflt-len (Array.repeat dflt-len &SetBucket.empty)))
 
-  (doc create-with-len "Create an empty set with a given number of buckets. Higher numbers decrease the probability of hash collisions while increasing the memory footprint.")
-  (defn create-with-len [len]
-    (init len (Array.repeat len &SetBucket.empty)))
+  (private put-!)
+  (hidden put-!)
+  (defn put-! [s v]
+    (let-do [idx (Int.positive-mod (hash v) @(n-buckets s))
+             b (buckets s)
+             n (Array.unsafe-nth b idx)]
+      (set-len! s (Int.inc @(len s)))
+      (SetBucket.push-back! n @v)))
 
-  (doc put "Put a a key k into the set s.")
-  (defn put [s k]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets &s))
-          ;; The lifetime system really doesn't like this function, had to put in a bunch of copying to make it compile:
-          ]
-      (update-buckets s &(fn [b]
-        (let [n (Array.unsafe-nth &b idx)]
-          (if (SetBucket.contains? n k)
-            b
-            (let [new-k @k] ;; HACK!
-              (Array.aset b idx (SetBucket.grow n new-k)))))))))
+  (doc resize "Resize a set `s` to size `size`.")
+  (defn resize [s size]
+    (let-do [n (init 0 size (Array.repeat size &SetBucket.empty))]
+      (for [i 0 @(n-buckets &s)]
+        (let [bucket (Array.unsafe-nth (buckets &s) i)
+              len (Array.length (SetBucket.entries bucket))
+              entries (SetBucket.entries bucket)]
+          (for [j 0 len]
+            (let [e (Array.unsafe-nth entries j)]
+              (put-! &n e)))))
+        n))
 
-  (doc put! "Put a a key k into the set s, in place.")
-  (defn put! [s k]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets s))
-          b (buckets s)
-          n (Array.unsafe-nth b idx)]
-      (when (not (SetBucket.contains? n k))
-        (SetBucket.push-back! n @k))))
+  (doc grow "Grow a set `s`. Should usually be handled automatically.")
+  (defn grow [s] (resize s (* @(n-buckets &s) 2)))
+  (doc shrink "Shrink a set `s`. Should usually be handled automatically.")
+  (defn shrink [s]
+    (let [new-size (/ @(n-buckets &s) 2)]
+      (if (< new-size dflt-len)
+        s
+        (resize s new-size))))
+
+  (doc contains? "Check whether the set `s` contains the value `v`.")
+  (defn contains? [s v]
+    (let [idx (Int.positive-mod (hash v) @(n-buckets s))]
+      (SetBucket.contains? (Array.unsafe-nth (buckets s) idx) v)))
+
+  (doc put "Put a value `v` into the set `s`.")
+  (defn put [s v]
+    (cond
+      (contains? &s v)
+        s
+      (> (/ (* @(len &s) 100) @(n-buckets &s)) min-load)
+        (put (grow s) v)
+      (let [idx (Int.positive-mod (hash v) @(n-buckets &s))
+            ;; The lifetime system really doesn't like this function, had to put in a bunch of copying to make it compile:
+            ]
+        (update-len
+          (update-buckets s &(fn [b]
+            (let [n (Array.unsafe-nth &b idx)]
+              (let [new-k @v] ;; HACK!
+                (Array.aset b idx (SetBucket.grow n new-k))))))
+          &Int.inc))))
+
+  (doc put! "Put a value `v` into the set `s`, in place.")
+  (defn put! [s v]
+    (unless (contains? s v)
+      (put-! s v)))
 
   (doc length "Get the length of set s.")
   (defn length [s]
-    (let-do [c 0]
-      (for [i 0 @(n-buckets s)]
-        (set! c (+ c (Array.length (SetBucket.entries (Array.unsafe-nth (buckets s) i))))))
-      c))
+    @(len s))
 
   (doc empty? "Check whether the set s is empty.")
   (defn empty? [s]
     (= (length s) 0))
 
-  (doc contains? "Check whether the set s contains the key k.")
-  (defn contains? [s k]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets s))]
-      (SetBucket.contains? (Array.unsafe-nth (buckets s) idx) k)))
-
-  (doc remove "Remove the key k from the set s.")
-  (defn remove [s k]
-    (let [idx (Int.positive-mod (hash k) @(n-buckets &s))]
-      (update-buckets s &(fn [b]
-                           (let [n (Array.unsafe-nth &b idx)]
-                             (Array.aset b idx (SetBucket.shrink n k)))))))
+  (doc remove "Remove the value `v` from the set `s`.")
+  (defn remove [s v]
+    (cond
+      (not (contains? &s v))
+        s
+      (> (/ (* @(len &s) 100) @(n-buckets &s)) min-load)
+        (remove (shrink s) v)
+      (let [idx (Int.positive-mod (hash v) @(n-buckets &s))]
+        (update-len
+          (update-buckets s &(fn [b]
+                               (let [n (Array.unsafe-nth &b idx)]
+                                 (Array.aset b idx (SetBucket.shrink n v)))))
+          &Int.dec))))
 
   (doc all? "Does the predicate hold for all values in this set?")
   (defn all? [pred set]
@@ -484,7 +566,7 @@
     (let-do [s (create)]
       (for [i 0 (Array.length a)]
         (let [e (Array.unsafe-nth a i)]
-          (set! s (put s e))))
+          (put! &s e)))
       s))
 
   (doc reduce "Reduce values of the set with function f. Order of reduction is not guaranteed")