Artificial Intelligence (AI) systems simulating human behavior are often called intelligent agents. We use the term epistemic state to refer to an actual or possible cognitive state that drives human behavior at a given point in time. This book provides three fundamental and generic approaches (logical, probabilistic, and modal) to representing and reasoning with agent epistemic states, specifically in the context of decision making. Each of these approaches can be applied to create the foundation for intelligent, decision-making software agents.