diff --git a/src/clj_fast/core.clj b/src/clj_fast/core.clj
index 10c2526..1a60f6e 100644
--- a/src/clj_fast/core.clj
+++ b/src/clj_fast/core.clj
@@ -99,16 +99,20 @@
   [v]
   `(Box. ~v))
 
+(defmacro as-box
+  [sym]
+  `(as clojure.lang.Box ~sym))
+
 (definline unbox!
   "Get `v` out of mutable box."
   [^Box b]
-  `(.-val ~(as Box b)))
+  `(.-val ~(as-box b)))
 
 (definline bset!
   "Sets the value of Box to `v` without regard to its current value.
   Returns `v`."
   [^Box b v]
-  `(set! (. ~(as Box b) val) ~v))
+  `(set! (. ~(as-box b) val) ~v))
 
 (definline bset-vals!
   "Sets the value of `b` to `v`. Returns `[old new]`, the values in the
diff --git a/src/clj_fast/instrument.clj b/src/clj_fast/instrument.clj
new file mode 100644
index 0000000..a93c3a1
--- /dev/null
+++ b/src/clj_fast/instrument.clj
@@ -0,0 +1,168 @@
+(ns clj-fast.instrument
+  (:require
+   [clj-fast.core :as c])
+  (:import
+   (java.util Queue)
+   (java.util.concurrent ConcurrentLinkedQueue)))
+
+(defonce
+  ^{:doc "All instrumented vars are registered in this set,
+  Allows for manipulation and querying of their metadata."}
+  instrumented-vars
+  (atom #{}))
+
+(def ^:private argvs
+  (reductions
+   conj
+   []
+   (map
+    (fn [n] (symbol (str "x" n)))
+    (range 20))))
+
+(defn- emit-instrumenting-expr
+  [before after]
+  (let [bodies
+        (map
+         (fn [argv]
+           `([~@argv]
+             (let [start# (~before)
+                   ret# (~'f ~@argv)]
+               (~after start#)
+               ret#)))
+         argvs)]
+    `(fn [~'f] (fn ~@bodies))))
+
+(comment
+  (emit-instrumenting-expr 'before 'after))
+
+(defmacro instrumenting
+  "Emit a function which when applied to another function will wrap it,
+  executing `before` before calling it, and `after` afterwards, with the
+  result of calling `before`.
+  Usage suggestion:
+  (alter-var-root v (instrumenting before after))"
+  [before after]
+  (let [b (gensym) a (gensym)]
+    `(let [~b ~before
+           ~a ~after]
+       ~(emit-instrumenting-expr b a))))
+
+(defn timing-before
+  "Nano time, suitable as a `before` function."
+  ^long [] (System/nanoTime))
+
+(defn timing-after-fn
+  "Nano time difference which will push into a Queue `q`, suitable as
+  `after` function, must take a Queue."
+  [^Queue q]
+  (fn [^long start]
+    (let [end (System/nanoTime)]
+      (.add q (unchecked-subtract end start)))))
+
+(defn instrument-var
+  "Wrap a var `v` with `f` and replace its meta with `m`"
+  ([v f]
+   (instrument-var v f nil))
+  ([v f m]
+   (alter-var-root v f)
+   (when (map? m)
+     (reset-meta! v m))
+   (swap! instrumented-vars conj v)
+   v))
+
+(defn instrument-var-fn
+  "Return a function for instrumenting a var with `before` and `after`.
+  `extra-meta` is a map with extra metadata which will be attached to the var."
+  ([before after]
+   (instrument-var-fn before after nil))
+  ([before after extra-meta]
+   (fn [v]
+     (let [old-meta (meta v)
+           r @v
+           m (merge old-meta {:old-root r :old-meta old-meta} extra-meta)]
+       (instrument-var v (instrumenting before after) m)))))
+
+(defn noop-fn [] (fn ([]) ([_])))
+
+(defn instrument-var-visitor
+  "Create an instrumentation function which registers visited vars in a
+  shared (atom set).
+  Useful for narrowing down the search space before causal profiling."
+  [a]
+  (fn [v]
+    (let [before (fn [] (swap! a conj v))
+          old-meta (meta v)
+          r @v
+          m (merge old-meta {:old-root r :old-meta old-meta})]
+      (instrument-var v (instrumenting before noop-fn) m))))
+
+(defn instrument-var-with-timing-fn
+  "Return a function which instruments a var with timing measurements in
+  nanosecods and appends the results to a Queue which will be attached
+  to the var's metadata."
+  []
+  (let [q (ConcurrentLinkedQueue.)]
+    (instrument-var-fn
+     timing-before
+     (timing-after-fn q)
+     {:queue q})))
+
+(defn sleep-cause-fn
+  [b]
+  (fn [] (Thread/sleep (long (c/unbox! b)))))
+
+(defn instrument-var-cause-fn
+  "Return a function which instruments a var for causal profiling.
+  The slow-down amount is saved in a box attached to the var's
+  metadata."
+  []
+  (let [b (c/box! 1)]
+    (instrument-var-fn
+     (sleep-cause-fn b)
+     (noop-fn)
+     {:box b})))
+
+(defn restore-var
+  "Restore var to its pre-instrumentation state."
+  [v]
+  (let [{:keys [old-root old-meta]}
+        (select-keys (meta v) [:old-root :old-meta])]
+    (alter-var-root v (constantly old-root))
+    (reset-meta! v old-meta)
+    (swap! instrumented-vars disj v)
+    v))
+
+(comment
+  (defn foo
+    [x]
+    (Thread/sleep 20)
+    (inc x))
+
+  ((instrument-var-with-timing-fn) #'foo)
+  ((instrument-var-cause-fn) #'foo)
+  (restore-var #'foo)
+
+  (dotimes [_ 1e3]
+    (foo 2))
+
+  (into [] (:queue (meta #'foo))))
+
+;;; Statistical analysis
+
+(defn square
+  ^long [^long x]
+  (unchecked-multiply x x))
+
+(defn mean
+  ^double [coll]
+  (double (/ (reduce unchecked-add coll) (count coll))))
+
+(defn stddev
+  [a]
+  (let [mn (mean a)]
+    (Math/sqrt
+     (/ (reduce #(+ %1 (square (- %2 mn))) 0 a)
+        (dec (count a))))))
+
+(comment
+  (- (mean (into [] (:queue (meta #'foo)))) 2e7))