The complex neurodegeneration underlying Alzheimer disease (AD), although incompletely understood, is characterised by an aberrant re-entry into the cell cycle in neurons. Pathological evidence, in the form of cell cycle markers and regulatory proteins, suggests that cell cycle re-entry is an early event in AD, which precedes the formation of amyloid-beta plaques and neurofibrillary tangles (NFTs). Although the exact mechanisms that induce and mediate these cell cycle events in AD are not clear, significant advances have been made in further understanding the pathological role of cell cycle re-entry in AD. Importantly, recent studies indicate that cell cycle re-entry is not a consequence, but rather a cause, of neurodegeneration, suggesting that targeting of cell cycle re-entry may provide an opportunity for therapeutic intervention. Moreover, multiple inducers of cell cycle re-entry and their interactions in AD have been proposed. Here, we review the most recent advances in understanding the pathological implications of cell cycle re-entry in AD.