The sites, gap levels, and migration barriers of interstitial H in Si are predicted. The hydrogenation of C-rich Si results in the formation of H2*(C) and C2H2, in contrast to FZ-Si where H2 molecules dominate. The fully saturated vacancy (VH4) also forms. This complex is normally stable up to 650 °C. However, in C-rich Si, VH4 anneals around 550 °C while the VH3HC complex appears. There, C replaces one of the four Si nearest-neighbors to the vacancy. This implies that VH4 begins to diffuse at 550 °C, and then traps at Cs. This in turn implies that all the VHn complexes (n = 1, 2, 3, 4) are mobile at moderate temperatures. In this paper, we discuss the energetics of H in Si, summarize the key experimental and theoretical results about H interactions in C-rich Si, and discuss the migration paths and activation energies of the four VHn complexes.