Part 1 introduces the foundational principles of well integrity as a dynamic, time-dependent challenge critical to the safe and sustainable operation of subsurface wells throughout their entire lifecycle. Beginning with a precise definition rooted in international standards (NORSOK D-010, ISO 16530-1), this chapter expands the concept to emphasize the evolving nature of integrity threats, including mechanical degradation, thermal cycling, and chemical deterioration of barriers. The chapter explores core pillars of well integrity—containment, control, structural soundness, and longevity—and examines how failures in these elements can manifest as sustained casing pressure, annular flow, or catastrophic blowouts. Through a detailed historical review of well integrity failures, including landmark events like the Ekofisk Bravo, Montara, and Macondo blowouts, the chapter traces the evolution of regulatory standards and industry practices, culminating in modern risk-based and lifecycle-oriented integrity frameworks. A compelling business case is presented, highlighting how integrity management enhances safety, environmental protection, economic performance, and corporate reputation. Furthermore, the chapter addresses how the energy transition reshapes integrity requirements in challenging applications such as carbon capture and storage (CCS), geothermal wells, and underground hydrogen storage (UHS), all of which impose unique thermal, chemical, and cyclic loading stresses on well components. The discussion underscores the necessity of adapting materials, monitoring systems, and lifecycle planning to new energy paradigms. Finally, the chapter introduces the concept of integrity touchpoints across the well lifecycle—from concept selection and design to abandonment—demonstrating how early decisions have long-term implications. Through engineering fundamentals such as stress–strain relationships and safety factor calculations, readers are equipped with the mechanical basis for analyzing barrier performance under complex downhole conditions. This foundational chapter sets the stage for the specialized well integrity challenges and solutions detailed in subsequent sections of the book.