This chapter explores the concept of AI agents—autonomous entities capable of perceiving their environment, making decisions, and taking actions to achieve specific goals. It begins by defining what constitutes an AI agent and outlines the principles involved in designing intelligent agents that balance reactive responses with goal-directed behaviour. Various types of agents are introduced, ranging from simple reflex agents to complex learning and planning agents, each suited to different tasks and environments. The chapter further examines multi-agent systems, where multiple agents interact and collaborate, often in dynamic and distributed settings. Advanced agent architectures are discussed, including those that support reasoning, learning, and adaptation. A special focus is given to agentic AI, which refers to systems with a high degree of autonomy and initiative, capable of pursuing long-term objectives and managing complex tasks. Finally, the chapter addresses key challenges in developing AI agents, such as scalability, coordination, ethical decision-making, and robustness in unpredictable environments.