This chapter highlights the interference theory and its evolution, and it offers some perspective on the status of the theory today. The major historical competitors to interference theory assign forgetting that is, observed performance losses to different stages. The decay theory attributes forgetting, to an actual weakening of the memory trace during the retention stage and the consolidation theory attributes forgetting to disruption of a lingering covert after-effect of the acquisition stage. There have been three major empirical disappointments: little or no evidence for spontaneous recovery has been obtained in modified modified free recall (MMFR) testing; predictions based on the interaction of single lists from the laboratory with prior natural-language habits have consistently failed to work out; and evidence on the pairwise specificity of interference and competition has been mixed at best. These findings have led to such revised notions about interference as the list-differentiation hypothesis of Underwood, the response-set suppression hypothesis of Postman, and the stimulus-encoding hypothesis of Martin.