Abstract. Opportunistic activity and context recognition systems draw from the characteristic to use sensing devices that just happen to be available rather than pre-defining a fixed sensor infrastructure at design time. Opportunistic sensing offers the possibility to obtain data from sensors that just happen to be available in the area surrounding the user. This enables users or applications to state recognition goals, saying what has to be sensed for, at runtime to the system. The available sensing devices that can contribute to the recognition goal are configured to an ensemble, which is the best set of sensors to recognize the goal. This paper describes the OPPORTUNITY Framework and shows its functionality with respect to four application cases (goal querying and sensor configuration, sensor appears/disappears, sensor learns from other sensor and sensor self trust) to show the dynamic nature of an opportunistic system as the available sensing infrastructure is not fixed and changes during runtime.