Real‐time ellipsometric characterization of the nucleation of hydrogenated amorphous silicon (a‐Si:H) prepared by plasma‐enhanced chemical vapor deposition (PECVD) on smooth, dense metal, and crystalline Si substrates is reviewed. The experimental results for photoelectronic quality a‐Si:H from pure SiH4 on Mo and Cr are consistent with the Volmer–Weber nucleation mode, with a separation of 40–50 Å between nucleation centers. For c‐Si substrates, a new interpretation suggests that nucleation occurs on the same scale, but the geometry in the first monolayer is disklike. A well‐defined lobe and cusp in the data can be ascribed to surface smoothening by diffusion that results upon coalescence of these structures. For films from a pure SiH4 plasma, the rates of coalescence and relaxation of substrate‐induced surface roughness on thick films are consistent with a diffusion length of ∼80 Å for the adsorbed precursors. For films prepared from a SiH4‐depleted plasma, the average surface diffusion length is reduced by at least a factor of 2. New results for the growth of μc‐Si:H on a‐Si:H films reveal an induction period of several minutes for the nucleation of microcrystallites. During this period a structurally defective 40–80 Å amorphous layer is deposited. Such a layer is expected to influence the properties of photoelectronic devices and compositionally modulated materials that use thin μc‐Si:H.