diff --git a/README.md b/README.md
index 66232d9..1f09d99 100644
--- a/README.md
+++ b/README.md
@@ -25,12 +25,24 @@ Any other mix works — it is all in `config.py`.
 ## Install
 
 ```bash
-pip3 install -r requirements.txt
+cd /home/pi/heos
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
 python3 app.py
 ```
 
 Then open `http://<pi-ip>:5005/`.
 
+The virtual environment is not optional on a current Raspberry Pi OS:
+`pip install` straight into the system Python is refused there with
+`externally-managed-environment`. It also keeps Flask out of the way of
+anything else running on the Pi.
+
+Every later `python3 ...` command here assumes the environment is active
+(`source .venv/bin/activate`); `deactivate` when you are done. The service
+below calls the environment's Python directly, so it does not care.
+
 ## Configure
 
 Open `http://<pi-ip>:5005/api/targets` and copy the exact `name` HEOS reports
@@ -70,7 +82,7 @@ Description=HEOS panel
 After=network-online.target
 
 [Service]
-ExecStart=/usr/bin/python3 /home/pi/heos/app.py
+ExecStart=/home/pi/heos/.venv/bin/python /home/pi/heos/app.py
 WorkingDirectory=/home/pi/heos
 Restart=always
 User=pi