A role for sleep in memory processes and neural plasticity has been suggested many times and in many different forms. However, we are far from a consensus on what this role might be and why it would be fulfilled preferentially by sleep. In this review, we distinguish between memory acquisition, consolidation, and maintenance, and we consider how sleep may specifically contribute to each of these phases. We also distinguish between declarative and nondeclarative memories and their relationships to different stages of sleep. Finally, we discuss whether different molecular and cellular aspects of neural plasticity may be associated preferentially with different behavioral states. A consideration of such molecular aspects could lead to more conclusive experiments concerning the relationship between sleep and plasticity.