This paper examines the hydrogen collision model of metastability in hydrogenated amorphous silicon nearly 5 years after its publication. A brief restatement of key model features and assumptions is followed by a review of recent, related, experimental and theoretical studies. Experimental tests and discoveries support several predictions and requirements of the model, including observation of (1) H pairing in the metastability final state, (2) a new timescale in defect creation, (3) an extremely high diffusion rate of mobile H and (4) room-temperature light-induced H motion. However, an experiment in a-SiGe:H casts doubt on the assumption of quasi-equilibration between the mobile H and dangling bonds. Theorists and modelers devised microscopic proposals for paired H in the metastability final state. but no entirely successful picture has emerged. (C) 2002 Elsevier Science B.V. All rights reserved.