A major development in understanding the structure and organization of memory was the identification of the medial temporal lobe memory system as one of the brain systems that support memory. Work on this topic began in the 1950s with the study of the noted amnesic patient H.M. and culminated in studies of an animal model of human memory impairment in the nonhuman primate. These discoveries opened new frontiers of research concerned with the functional specialization of structures within the medial temporal lobe, the existence of multiple memory systems, the process of memory consolidation, and the role of neural replay and sleep in the consolidation process. This work also led to new insights about how and where memories are ultimately stored in the brain. All of this research has improved our understanding of how memory is affected by normal aging and why it is so profoundly impaired by the pathological processes associated with dementia.