The hippocampal formation has long been the focus of intense interest among neuroscientists. The discovery by Scoville and Milner (1957) that the mesial temporal lobe, including the hippocampal formation, played a central role in memory function in humans can be said to have started the modern era of research on this brain system. There followed two decades of animal research aimed at creating experimental models of the memory defects observed in humans, including my dissertation research, which met with scant success (Nadel, 1968). Indeed, when John O’Keefe and I started considering the hippocampus and its possible role in spatial mapping several years later, one of our greatest challenges was to analyze the extant lesion literature in terms of this theory with the minimum amount of special pleading. There were already several hundred such studies, varying extensively in terms of exact size and regional location of the lesion, the means by which the lesion had been created, the kind of animal used, the nature of the task, the kind of motivation employed, and so on. Yet, it was our feeling in 1976, when we terminated the review of the lesion literature, which was ultimately published in The Hippocumpiis CIS a Cognitive