diff --git a/esp8266/README.md b/esp8266/README.md
index 0509a3799b5f35004d9a1d1207acf0dd1130c1c2..5965fa4bce14e94f76de23e7e39bcfa58dee7cf1 100644
--- a/esp8266/README.md
+++ b/esp8266/README.md
@@ -39,6 +39,12 @@ $ git submodule update --init
 See the README in the repository root for more information about external
 dependencies.
 
+The MicroPython cross-compiler must be built to pre-compile some of the
+built-in scripts to bytecode.  This can be done using:
+```bash
+$ make -C mpy-cross
+```
+
 Then, to build MicroPython for the ESP8266, just run:
 ```bash
 $ cd esp8266