Configuration of user interfaces is described. In an implementation, a method includes receiving a query from a device. The query is for a user interface (UI) from a client. The UI, when executed by the device, is configured to form a request for execution of a control function by the client. A processing of the request determines whether the execution by the client of the control function is permitted. The processing is performed at a head end by using client state data. The hardware and/or software capabilities of the device to execute the UI are detected. The UI is configured based on the hardware and/or software capabilities of the device.