Situated multi-agent systems are MAS with an explicit representation of the environment, which agents can observe and in which agents can act. An appropriate middleware platform should support the construction and manipulation of such an environment. We require three properties from the environment: (1) observableness: an agent must be able to get a current view on its context; (2) activity: the environment updates the agent?s current view; (3) dynamism: the environment can change without direct in- fluence from the agents. ObjectPlaces is a tuplespace-like middleware that offers an event-based interface and a concept of dynamic objects to support these requirements.