The hippocampus and cerebral neocortex receive massive cholinergic projections from the basal forebrain. These projections arise predominantly within the nucleus basalis of Meynert, also known as the Ch4 cell group. The cholinergic projection of the cerebral cortex plays a major role in sustaining episodic memory, attention, and experience-induced neuroplasticity. Progressive neurofibrillary degeneration in the nucleus basalis can be seen throughout the continuum that leads from normal aging to mild cognitive impairment and Alzheimer’s disease. The resultant perturbation of cortical cholinergic neurotransmission may contribute to the cognitive changes seen in normal aging and Alzheimer’s disease.