diff --git a/ZMQ-Ports.md b/ZMQ-Ports.md
new file mode 100644
index 0000000..89c971a
--- /dev/null
+++ b/ZMQ-Ports.md
@@ -0,0 +1,25 @@
+# Intorduction
+
+PixelatorController communicates with clients (e.g. PixelatorGUI) via two ZMQ ports:
+- Publisher: PixelatorController broadcasts scan data and positioner updates.
+- Request: 2-way communication channel for clients to request info or actions from PixelatorController.
+
+The port numbers used for these communication channels has to match in the configuration files for both PixelatorController and any clients.
+
+# PixelatorController
+
+The settings file that is passed as an argument to the startup command must include the `publisherPort` and `requestPort` fields. For example:
+```json
+"publisherPort" : "55561",
+"requestPort" : "55562",
+```
+
+# PixelatorGUI
+
+The configuration of PixelatorGUI is in `build/PixelatorGUI/etc/PixelatorGUI.ini`. The `Resouorce` section must contain the `PIXELATORCONTROLLER_PORT_PUBLISHER` and `PIXELATORCONTROLLER_PORT_REQUEST` fields. The values of these fields **must match** the corresponding fields in the PixelatorController configuration. Further, the value of the `PIXELATORCONTROLLER_HOSTNAME` field must match the hostname of the machine that PixelatorController is running on. For example:
+```
+[Resource]
+PIXELATORCONTROLLER_HOSTNAME="stxm-1.psi.ch"
+PIXELATORCONTROLLER_PORT_PUBLISHER=55561
+PIXELATORCONTROLLER_PORT_REQUEST=55562
+```
\ No newline at end of file