This chapter introduces the barrier philosophy as the cornerstone of well integrity management, focusing on the principles of containment across the entire well lifecycle. Central to this philosophy is the systematic design, verification, and maintenance of well barrier elements (WBEs) and their integration into well barrier envelopes to prevent uncontrolled fluid or pressure migration. This chapter provides a rigorous definition of WBEs and barrier envelopes, emphasizing their functional roles, reliability requirements, and the physical and operational conditions they must endure. The chapter elaborates on the two-barrier principle, mandating at least two independent, verifiable barriers between any pressure source and the environment to ensure redundancy and reduce the risk of total containment failure. Key engineering concepts—including pressure gradients, hydrostatic calculations, annular pressure build-up, and probabilistic reliability models—are introduced to support the evaluation of barrier performance. Case-specific configurations for production, drilling, intervention, and abandonment phases are illustrated, highlighting the evolving nature of primary and secondary barriers across operational contexts. The importance of independence and verifiability in barrier design is addressed in depth, detailing how shared failure modes, inadequate testing, or poor documentation can compromise integrity. The chapter also introduces Well Barrier Schematics (WBS) as vital visual tools for risk communication, operational planning, and compliance. Standardized WBS enhance understanding of barrier status, verification history, and potential leak paths, facilitating safer decision-making. Finally, the application of the barrier philosophy across design, construction, production, and decommissioning phases is detailed, illustrating its adaptability and relevance in both conventional and evolving energy systems. By grounding the discussion in international standards (e.g., ISO 16530, NORSOK D-010, API RP90) and engineering practice, this chapter establishes the foundational mindset and technical framework necessary for robust containment assurance in all well operations.