Atmospheric water harvesting (AWH) technologies promise sustainable solutions to mitigate global water scarcity by extracting moisture from the air. This chapter explores the fundamental thermodynamics governing the AWH systems, focusing on key processes involving liquid–vapor phase change. The analyses provide an overview of the phase equilibrium of water and psychrometric principles related to AWH technologies. Different AWH technologies, including refrigeration-based and adsorption-based systems, are investigated through the lens of thermodynamic principles. Performance metrics such as water harvesting rate and unit power consumption are also discussed. By integrating theoretical principles with practical considerations, the present chapter provides a framework for analyzing sustainable and energy-efficient AWH systems, highlighting future challenges and opportunities in this emerging field.