The influence of humidity on semiconductor device reliability is investigated with two main purposes: to emphasize the role of humidity in the failure process as a stress factor and to model the reliability-humidity relationship. Experiments were performed on two types of plastic encapsulated semiconductor devices (optoelectronic and ICs). Reliability data obtained from functioning at humidity and temperature are compared with those obtained from functioning at the same temperature but at a lower humidity level. The role of humidity as a stress factor becomes obvious when the specific failure mechanisms induced are identified. The experimental data are also used to validate an original model, a generalized Arrhenius relation. The model allows the design of accelerated tests with humidity as one of the stress factors. The model is compared with previous models.