This entry examines the concept of “mechanism,” with a particular focus on complex neurocognitive mechanisms as they are used in contemporary cognitive neuroscience and cognitive science. It begins with an introduction that outlines the various uses of the term in different contexts. Next, it offers a historical overview of significant uses of the concept in philosophy and science. The discussion then turns to the notion of mechanism and mechanistic explanation, contrasting these with deductive-nomological explanations. Key debates concerning these concepts and theories of explanation within cognitive science are analyzed. Finally, the entry addresses issues of reduction and autonomy as they relate to neurocognitive mechanisms and mechanistic explanations, and it presents potential directions for future developments in the concept and its role within contemporary theoretical frameworks in cognitive science.