Theoretical arguments and empirical data are presented in favor of the hypothesis that the hippocampal system supports a declarative memory capacity in animals as well as humans. This view is advanced by identifying two prominent characteristics of human declarative memory and by operationalizing and evaluating them using both experimental lesion and single unit recording studies on animals. First, hippocampal processing is not selective to any particular category of learning materials; instead, it supports comparisons among all kinds of information in memory, resulting in a representation of critical relations between items. Conversely, individual representations are supported outside the hippocampal system. Second, hippocampal-dependent, relational memory representations involve a flexible organization that permits inferences from memory in novel situations. Conversely, hippocampal-independent individual representations can support only repetition of procedures acquired during original learning. Correspondences between the neuropsychological and neurophysiological findings presented serve to indicate how these properties of hippocampal representation support declarative memory across behavioral paradigms and across species.