This paper presents a general theory of action in multiagent systems which rely on a clear distinction between influences, which are produced by agents’ behaviour, and the reaction of the environment. This theory allows us to rigorously describe complex interactions between situated agents. Two kind of interactions are investigated: interactions between an agent and its environment, and interactions between agents through the environment. In this respect, we will show that the second kind is a special case of the first one. Although many theories about action are based on mental issues, we rely mainly on "what happens" in the world, i. e. the transformations that take place during interaction. Therefore, as shown in this paper, this theory is general enough to deal with both tropistic (reactive agents) and hysteretic (agents with memory) agents as special cases of general multiagent systems.