This chapter addresses the foundational role of casing and tubing systems in maintaining long-term well integrity, emphasizing their function as both structural and pressure-containment barriers. It presents a comprehensive framework for the mechanical and environmental design of well tubulars, incorporating industry standards (e.g., API 5CT) and accounting for lifecycle load scenarios including burst, collapse, tension, compression, and triaxial stresses. The chapter begins by introducing the mud window as a guiding constraint for casing depth selection and progresses to detailed analytical models for stress evaluation under downhole conditions. Material selection strategies are discussed in the context of temperature effects and sour service environments (e.g., H₂S), with particular focus on grade-specific performance and degradation. Additionally, the integrity of tubular connections is rigorously examined, comparing API and premium designs with respect to sealability, mechanical robustness, and fatigue resistance. Design factors and safety margins are highlighted to address uncertainties, manufacturing tolerances, and operational variability. Finally, casing wear mechanisms and predictive models are introduced to safeguard against progressive damage in deviated wells. Through this integrated approach, the chapter equips engineers with the critical design principles and calculations needed to ensure tubular reliability across drilling, production, and abandonment phases.