diff --git a/README.md b/README.md
index 5d1e908db1bbd010082cbfa980d897c6604b85c5..43f08b455acd3492872ec3d039a9390fcd317b2c 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,19 @@ Note this is not yet complete.
 go get -u gitlab.com/lyda/gqgmc/cmd/gqgmcd
 ```
 
+## Prometheus variables
+
+There is a sample `prometheus.yml` configuration in the `docs/`
+directory.  See the Prometheus
+[getting started](https://prometheus.io/docs/introduction/getting_started/)
+docs for more information.
+
+* `gqgmc_geiger_cpm` (histogram) CPM readings.
+* `gqgmc_geiger_cps` (histogram) CPS readings.
+* `gqgmc_power_volts` (histogram) Voltage readings.
+* `gqgmc_sys_errors` (histogram) Error counts. Records problems
+  communicating with the device.
+
 ## Configuring udev
 
 Copy the `linux/51-gqgmc.rules` file to the `/etc/udev/rules.d`