In multi-agent domains, an agent's action may not just change the world and the agent's knowledge and beliefs about the world, but also may change other agents' knowledge and beliefs about the world and their knowledge and beliefs about other agents' knowledge and beliefs about the world. Similarly, the goals of an agent in a multi-agent world may involve manipulating the knowledge and beliefs of other agents' and again, not just their knowledge 1 about the world, but also their knowledge about other agents' knowledge about the world. The goal of this paper is to present an action language, called mA+, that has the necessary features to address the above aspects in representing and reasoning about actions and change in multi-agent domains.This action language can be viewed as a generalization of the single-agent action languages extensively studied in the literature, to the case of multi-agent domains. The language allows the representation of and reasoning about different types of actions that an agent can perform in a domain where many other agents might be present-such as world-altering actions, sensing actions, and announcement/communication actions. The action language also allows the specification of agents' dynamic awareness of action occurrences which has future implications on what agents' know about the world and other agents' knowledge about the world. The language mA+ considers three different types of awareness: full awareness, partial awareness, and complete oblivion of an action occurrence and its effects. This keeps the language simple, yet powerful enough to address a large variety of knowledge manipulation scenarios in multi-agent domains.The semantics of the language relies on the notion of state, which is described by a pointed Kripke model and is used to encode the agent's knowledge and the real state of the world. The semantics is defined by a transition function that maps pairs of actions and states into sets of states. The paper illustrates properties of the action theories, including properties that guarantee finiteness of the set of initial states and their practical implementability. Finally, the paper relates mA+ to other related formalisms that contribute to reasoning about actions in multi-agent domains.