diff --git a/Env.md b/Env.md
new file mode 100644
index 0000000..4981285
--- /dev/null
+++ b/Env.md
@@ -0,0 +1,38 @@
+## Env
+
+![Env](images/env.png)
+
+Env is a combination pitch detector and envelope follower. Given an input audio signal,
+Env tries to detect the loudness and musical pitch of a single tone in it.
+
+Env is based on the pitch detector and envelope follower in the Surge XT [TreeMonster](https://library.vcvrack.com/SurgeXTRack/SurgeXTFXTreeMonster) module. Thanks to [Paul Walker](https://github.com/baconpaul/) for ideas and encouragement in this project!
+
+## Controls
+
+There are 4 controls for Env: THRESH, SPEED, FREQ, and RES.
+These controls assist making the pitch detector work better on a variety of possible inputs.
+From left to right, each control consists of a CV input port,
+a smaller attenuverter knob, and a larger control knob.
+
+* **THRESH**: The amplitude threshold of the input signal needed to trigger pitch detection. The range is −94 dB to 0 dB, with a default of −24 dB. Adjust as needed to report pitch for valid notes while rejecting any low-level noise while notes are not playing.
+* **SPEED**: How quickly to slew reported output pitches. Lower values result in more stable note detection, but with a trombone-like glide through note changes. Faster values track changes in notes more quickly, but is more susceptible to unwanted variations of pitch.
+* **FREQ**: Adjusts the center frequency of a bandpass prefilter that helps narrow in on the intended pitch range of the notes being detected. This can help reject unwanted harmonics from the input audio.
+* **RES**: Adjusts the resonance of the prefilter. Higher resonance can help squeeze the passband closer to the expected range of notes in the input audio. Too high a value can cause erroneous detection of notes at or near the center frequency.
+
+## Audio Input
+
+The AUDIO input port receives a cable with one or more channels of audio signal.
+Each channel in a polyphonic cable is processed as a completely independent audio signal.
+
+## Env Output
+
+The ENV output port carries an amplitude signal that follows the overall amplitude of
+the input audio. The ENV port has as many channels as the input port, with a separate
+envelope factor for each channel of the input AUDIO port.
+
+## V/OCT output
+
+The V/OCT output reports the pitch of any detected signal. The zero volt level indicates
+a C4 note (261.625 Hz). Each unit volt indicates an octave. If no pitch can be detected,
+this port may output −10 V as a placeholder. Like the ENV port, V/OCT is polyphonic;
+there will be one channel of output for each channel of input on the AUDIO port.
diff --git a/README.md b/README.md
index 198f175..744b8c0 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,16 @@ hexagonal mesh of balls and springs.
 
 ---
 
+## Env
+
+A polyphonic envelope follower and pitch detector.
+
+<a href="Env.md">Sapphire Env documentation</a>
+
+<a href="Env.md"><img src="images/env.png" /></a>
+
+---
+
 ## Frolic
 
 A chaotic oscillator for generating 3 simultaneous smooth but unpredictable voltages.