An overview of the various ways in which hydrogen (H) interacts with different forms of silicon (Si) is presented. First‐principles theory has contributed significantly to our understanding of these phenomena. For crystalline silicon, the discussion encompasses H in solid solution, H diffusion, H interactions with deep and shallow levels, and H on the surface. The structure and energetics of Si‐H bonds are relevant for modeling the role of hydrogen in amorphous silicon. For polycrystalline silicon, the interactions of hydrogen with grain boundaries and, in particular, the insertion of H into strained Si‐Si bonds are addressed. A final topic concerns the role of hydrogen in silicon‐based layered structures, which play a role in light emission from porous silicon.