High quality a-Si:H films are deposited by d.c. magnetron reactive sputtering of a Si target in an (Ar + H2) plasma. This paper reports the first comprehensive understanding of the growth process. The incident flux, surface H coverage, H2 release, and bulk H incorporation are determined using four in situ, real time techniques: double modulation mass spectroscopy, isotope replacement experiments, reflection absorption infra-red spectroscopy, and spectroscopic ellipsometry. In addition, the sputtered particle transport is simulated using Monte-Carlo techniques. For conditions which produce electronic quality a-Si:H, the total H flux arriving at the surface varies between 0.5 – 2 times the depositing Si flux; approximately half of this flux appears to reflect from the surface without interaction. The growth surface has excess H varying between 0.5 – 2 × 1015/cm2, and this surface H coverage is uniquely related to the bulk H incorporation.