This paper describes the outcome of a study into the feasibility of a reliability circuit simulator for ICs in general and the critical parameters involved in particular. The necessary conditions are formulated that have to be fulfilled before any construction of the reliability simulator is meaningful or can be done at all. It has been found that failure mechanisms in the wear-out regime meet these conditions. Next, a general approach is given to make a simulator. This approach is actually derived from circuit simulator activities on hot carrier degradation and electromigration, respectively. Finally, the different groups of parameters are defined.