High demands on cognitive functions in daily life call for an efficient memory system that reduces interference between memories and enables generalizations across similar events. By means of pattern separation, similar memories are stored as distinct, non-overlapping representations, whereas during pattern completion, previously stored memories are reactivated by partial environmental cues. These two functions are critically reliant on the hippocampus. Evidence from computational models, studies in rodents, as well as human data support the idea that pattern separation and completion are (...) mediated by the hippocampal dentate gyrus and CA3 regions. However, studies in humans lack information regarding mechanistic aspects of causality regarding the anatomical structures of the hippocampus and pattern separation and completion processes. The aim of this thesis was to elucidate the role of the human hippocampus and its subfield-specific contributions to pattern separation. We examined natural lesion models, by means of selective CA1 lesions during a transient global amnesia, and in a rare form of limbic encephalitis, where neurodegeneration preferentially shows in the dentate gyrus and CA3. The results showed that pattern separation measured by a mnemonic similarity task was best predicted by the volume of the dentate gyrus, whereas recognition memory was stronger associated with the volume of CA1. We also found a strong deficit in pattern separation associated with selective CA1 lesions. We then examined pattern separation performance after post-encoding sleep in healthy humans to clarify the neurobiological processes of memory consolidation. We demonstrated the relevance of hippocampal information processing during sleep in the stabilization of separated information. This might also suggest a link between pattern separation and completion processes during sleep-dependent memory consolidation. (shrink)