Large language model (LLM) agents are transforming the landscape of artificial intelligence, enabling sophisticated interactions, reasoning, and autonomous decision. This chapter explores the theoretical and practical foundations of LLM agents and multi-agent systems, highlighting their architecture, communication strategies, and coordination mechanisms. The chapter also presents examples of LLM agent applications in software engineering, demonstrating their impact on tasks such as code generation, debugging, and automated software development. Finally, the chapter describes the programming paradigms for designing and deploying intelligent agents and multi-agent coordination frameworks. By summarizing key advancements and methodologies, this chapter provides a comprehensive guide to developing and understanding LLM agents and multi-agent systems in modern AI applications.