In order to implement reactive and proactive functionalities in a pervasive environment, contextual data must be processed. One of the most important features of the context is the position of the users and the devices. In this paper, we describe a method to determine the position of a WiFi enabled device. The prediction is based on the signal strength of the available access points. The prediction model is built from a database containing the signal strength measured in some known locations. The result is the name of the room/office where the device is localised. We also present a usage scenario, in which the user/device position is used to start proactive actions in our pervasive service environment called PerSE