The current understanding of reliability issues for InP HEMTs is reviewed. To date, the origin of some instability and degradation phenomena have been identified and the solutions to eliminate or mitigate them have been found. On the other hand, some degradation phenomena are quite complicated and are still under investigation. The increase of drain resistance is one of them. This might be related to the hot electron effect, but it is still an open question as to where and how it happens. Some efforts at solving this mystery, including cathodoluminescence studies, are presented. (c) 2006 Elsevier B.V. All rights reserved.