This chapter takes a close look at phase change materials (PCMs) and their role in latent heat thermal energy storage systems. It explains how melting and solidification processes allow PCMs to absorb and release heat under nearly constant temperature conditions. The chapter discusses important thermophysical properties that affect PCM performance, such as heat of fusion, melting temperature, thermal conductivity, and chemical stability. It also covers criteria for choosing effective PCMs and addresses common issues like low thermal conductivity, subcooling, and phase segregation. The chapter describes methods for testing thermal stability, including repeated cycling and differential scanning calorimetry (DSC). It offers a clear classification of PCMs into organic, inorganic, and eutectic types, highlighting their benefits and drawbacks. The chapter reviews applications of PCMs in electronics, building systems, solar energy, and refrigeration. It concludes by stressing the need for material innovation to improve thermal performance and ensure long-term reliability in practical energy storage systems.