Many instances of commonsense reasoning involve the mental states of agents. An agent is an entity that performs purposeful actions in the world in which it exists. Examples of agents are people, animals, organizations, robots, and computer programs. To interpret an agent’s actions and make predictions about an agent’s future actions, we must model the mental states of agents. We address agent behavior, which requires the modeling of beliefs, goals, and plans. We discuss the modeling of emotions. We describe the epistemic functional event calculus for reasoning about knowledge and action.