The incorporation of hydrogen during film growth and the role that hydrogen plays in determining the amorphous silicon structure are discussed in the chapter. The chapter reviews the evidence for the existence of bonded hydrogen in amorphous silicon. Hydrogenated amorphous silicon displays various metastable conductance changes, which are attributed to the motion of bonded hydrogen. Measurements of the hydrogen diffusion coefficient and the various models proposed to account for the diffusion data are discussed in the chapter. A recent proposal that the hydrogen in amorphous silicon can be considered as to separate subnetwork with glass-like properties is described in the chapter to account both for the defect creation data and the defect removal during film growth. The diffusion of molecular H is minor compared to that of atomic H. The lowest intrinsic defect densities are found in hydrogenated amorphous silicon samples prepared by the radio-frequency glow-discharge decomposition of silane.