This chapter looks at how different memory systems are influenced by sleep. It describes the currently most-widely accepted model of consolidation of hippocampus-dependent memory. The chapter also looks at human functional magnetic resonance imaging (fMRI) studies which provide evidence that, in fact, memories are re-activated, re-organized, and re-processed during sleep. Reactivation occurs during post-learning sleep, and it seems to be an important component of memory consolidation. In general, it has been found that it occurs in those brain regions most strongly related to the specific learning task. Re-activation could therefore support synaptic consolidation of memory traces. However, recent studies also provide more and more evidence for systems memory consolidation. Looking for signs of re-processing during sleep is the most difficult to do, because based on imaging data alone it is hard to distinguish from re-organization, and there are only few behavioral tasks that are designed to examine such changes.