This chapter introduces the core principles of the agile approach, which was originally formulated by seventeen software practitioners in February 2001. The Agile Manifesto, as presented in Chapter 3, emphasizes human, organizational, and technological aspects of software development processes. It advocates for individuals and interactions over processes and tools, and working software over comprehensive documentation. These principles guide agile teams to prioritize people, communication, and the creation of high-quality software products. The chapter also discusses how the Agile Manifesto can be applied to various agile methods such as Extreme Programming, SCRUM, Lean, DSDM, Adaptive Software Development, and Crystal.This chapter introduces the core principles of the agile approach, which was originally formulated by seventeen software practitioners in February 2001. The Agile Manifesto, as presented in Chapter 3, emphasizes human, organizational, and technological aspects of software development processes. It advocates for individuals and interactions over processes and tools, and working software over comprehensive documentation. These principles guide agile teams to prioritize people, communication, and the creation of high-quality software products. The chapter also discusses how the Agile Manifesto can be applied to various agile methods such as Extreme Programming, SCRUM, Lean, DSDM, Adaptive Software Development, and Crystal.