A selected survey of the phenomenon of light-induced deep defect creation in the hydrogenated amorphous silicon-germanium is presented. First a general review of the early studies that established the key salient features of light-induced degradation in a-Si,Ge:H is given. This is followed by a discussion of a couple of complicating issues that have more recently come to light; namely, the possibility that charged defects play a more central role in the alloys, and that both Si and Ge metastable dangling bonds may be playing a significant role in the alloys with germanium fractions below 20 at%. Following this, the results of some recent studies are summarized that have been focusing on the details of degradation in the low Ge fraction alloys to gain insight into the fundamentals of degradation of amorphous silicon materials in general. This review concludes with an overall assessment of the level of our understanding of degradation in the a-Si,Ge:H alloys and where some key issues are still remaining to be resolved. (C) 2002 Elsevier Science B.V. All rights reserved.