Two possible models of forgetting in a short-term memory task subject to proactive interference are considered. One model assumes that the fundamental forgetting process is competition while the other relies upon a decay of the memory trace. Both models are shown to provide plausible, quantitatively accurate explanations of forgetting in the Brown-Peterson situation. Two general classes of models are defined, and some data are presented which deny the feasibility of the class which contains the decay model as a special case. A tentative model, based on the assumption that retrieval depends upon temporal discriminations and a “filtering” of traces is proposed.