This chapter provides a thorough examination of corrosion and erosion phenomena that compromise well integrity throughout the lifecycle of oil and gas wells. It begins with the fundamental principles of electrochemical corrosion, emphasizing the roles of anodic and cathodic reactions, electrolytes, and metallic pathways in downhole environments. The chapter then explores internal corrosion mechanisms driven by corrosive species such as CO₂, H₂S, O₂, microbiological agents, and organic acids, detailing their electrochemical pathways, scale formation and breakdown, and specific degradation forms like pitting, hydrogen-induced cracking, and sulfide stress cracking. External corrosion processes affecting the outer surfaces of casing strings due to formation fluids, cement pore water, and environmental exposure are also examined. The synergistic impact of erosion and erosion-corrosion is discussed, highlighting how high-velocity flow and entrained solids exacerbate metal loss beyond the sum of their individual effects. Recognizing these threats, the chapter presents a suite of corrosion control strategies including material selection, use of corrosion-resistant alloys (CRAs), chemical inhibition, protective coatings and linings, operational controls, and cathodic protection. It also evaluates the strengths and limitations of various corrosion monitoring techniques such as corrosion coupons, electrical resistance probes, and inline inspection tools. Given the challenges of predicting localized corrosion, ensuring inhibitor effectiveness in multiphase flow, and accessing downhole regions, this chapter emphasizes an integrated, risk-based approach to corrosion management. By consolidating industry standards (API, ISO, NORSOK) and case-specific mitigation practices, it equips engineers and asset managers with the tools to proactively address time-dependent metal degradation and safeguard wellbore structural integrity in increasingly aggressive operational environments.