This chapter provides an overview of the fundamentals of the agent-oriented paradigm and of Multi-Agent Systems (MASs). Specifically, the first section introduces the agent-based paradigm and comments its main features. The second section discusses multi-agent systems and their main features by focusing on the concepts of autonomy, coordination, norms, and emerging behavior that jointly characterize agents and MASs. Finally, in the third and fourth sections, the two main approaches to use the agent-oriented paradigm to develop agent-based simulations and agent-based software systems are discussed by analyzing the leading and most renowned reference software platforms.