 In the last two decades the notion of an intelligent agent has emerged as a unifying concept to discuss the theory and practice of artificial intelligence (cf. Russell and Norvig 1995, Nilsson 1998). In short, agents are software entities that display forms of intelligence/rationality and autonomy. They are able to take initiative and make decisions to take action on their own without direct control of a human user. In this subsection we will see how logic (of action) is used to describe / specify the (desired) behavior of agents (cf. Wooldridge 2002). First we focus on single agents, after which we turn to settings with multiple agents, called multi-agent systems (MAS) or even agent societies.