In the human brain, functionally and anatomically defined systems exist for actively encoding, consolidating, and retrieving memories of experiences (episodic memory); accumulating and accessing factual information in a body of knowledge (semantic memory); and processing and manipulating information (working memory). These three declarative memory systems can be distinguished from other nondeclarative memory systems such as procedural learning and priming.^1–4 Brain-behavior studies using a variety of approaches, from lesion-based research to functional magnetic resonance imaging (fMRI), demonstrate distinct, though interrelated, neural circuitry for working, episodic, and semantic memory.^4,5 Each of these three memory systems is affected somewhat differently by aging and dementia. In this chapter, the episodic, semantic, and working memory systems will be considered in turn, with special attention to changes associated with aging and with memory disorders such as Alzheimer’s disease and Mild Cognitive Impairment.