The discovery of declarative memory as distinct from other forms of memory is a major recent achievement in cognitive science. Basic issues about the nature of declarative memory are considered in this review from the perspective of studies on its underlying brain mechanisms. These studies have shown that declarative memory is mediated by a specific brain system including areas of the cerebral cortex and hippocampal region that make distinct functional contributions to memory processing. These processing mechanisms mediate the organization of memories in ways that can support the special properties of declarative or explicit memory expression. Furthermore, the basic properties of declarative memory in human beings can be viewed as evolving from a capacity for organized memory representation and flexible memory expression in animals.