Implicit memory reflects itself on situations in which previously acquired information is expressed, without awareness or intention. The study of implicit memory has had a profound impact on how researchers have investigated the human memory. In this paper, we review the main studies which have revealed dissociations between direct and indirect tests of memory, and address the two main theoretical approaches used to explain these dissociations: the processing view and the multiple systems view. We then describe recent neuroscience studies and discuss its contributions to help clarify the debate about the mechanisms involved in explicit and implicit memory. Finally, we critically analyze some questions and controversies surrounding this literature, such as (a) the transparency assumption and the contamination issue; and (b) the theoretical utility of the dissociations. We emphasize that the biggest challenge for future research is to develop comprehensive theories that integrate behavioral and neuroscience findings.