Research on situated multiagent systems (situated MAS) investigates decentralized architectures for software systems that have to deal with highly dynamic operating conditions. To realize the system requirements, the agents of a situated MAS have to coordinate their behavior. The agent environment provides a means to mediate (i.e., enable and constrain) agents' activities in the system. Laws embedded in the agent environment allow to define application specific constrains on agents activities. In this paper, we declaratively specify the semantics of laws for perception, action, and communication in situated MAS. We illustrate the laws with concrete examples in an automated transportation system that we have developed. Mediation of agents' activities via the agent environment improves separation of concerns in MAS and helps to manage complexity, especially in open and pervasive environments.