Hydrogen plays a critical role in CMOS technology, passivating and neutralizing defects. CMOS technology would be slower, less capable, and less reliable without the hydrogen mechanisms that passivate defects. In addition, hydrogen plays a significant role in process and plasma damage, confounding the data analysis and problem-solving. Typically, a large fraction of process damage problems are observed interacting with hydrogen processing, shifting electrical parameters, and changing the impact of process damage. Exposure to hydrogen can partially recover the impact of plasma damage, hiding the damage and creating the potential for latent damage, creating a reliability problem. This chapter examines the complex hydrogen role in CMOS manufacturing, including hydrogen motion in the wafer, hydrogen passivation, interaction with oxygen and dopants, interaction with thermal donors, interaction with contaminants and metals, physical defects, hydrogen barriers, hydrogen emission from films, deuterium, and the impact on radiation hardness.