In this paper we describe an implicit user interface for smart environment control: We make our system guess how to assist the user(s) proactively. Our controller is based on two formal descriptions: One that describes user activities, and another that specifies the devices in the environment. Putting both together, we can synthesize a probabilistic model, the states of which resemble activities performed by the user(s) and are annotated with sequences of device actions, with the latter to be executed in cases particular activities have been recognized. The resulting system is purely reactive and can be executed in real time. Categories and Subject Descriptors H.5 [User Interfaces]: Input Devices and Strategies General Terms Theory Keywords intention recognition, HMM, planning, smart environments