This chapter presents a comprehensive overview of primary cementing as a foundational operation in well construction, emphasizing its critical role in establishing long-term zonal isolation and structural well integrity. The discussion begins with the core functions of well cement—including hydraulic sealing, mechanical support, corrosion protection, and well control—and proceeds to explore the chemistry of Portland cement, hydration reactions, and the extensive use of additives to tailor slurry performance for downhole environments. It details key design principles for slurry formulation, covering essential parameters such as density, rheology, thickening time, fluid loss, strength development, shrinkage/expansion, and gas migration resistance. The chapter emphasizes the importance of proper mud removal, the use of spacers and centralizers, and dynamic cementing techniques to ensure effective bonding between the cement, casing, and formation. It outlines hydraulic and rheological modeling frameworks, including pressure drop equations, gel strength development, and displacement efficiency metrics. Practical cement placement strategies are analyzed in the context of preventing gas migration, ensuring proper plug sequencing, and maintaining hydrostatic control. Finally, the chapter addresses the cement setting and curing process, WOC timing, bond strength development, and the implications of shrinkage and degradation. Through detailed equations, field practices, and case-referenced insights, the chapter equips engineers with the critical knowledge needed to design and execute successful primary cementing operations that prevent sustained casing pressure, formation crossflow, and long-term integrity failures.