Neuroimaging methodologies have shown some of their greatest promise in studies of Alzheimer's disease (AD). Imaging outcome measures are now entering the arena of investigational trials for AD, to elucidate treatment mechanisms, optimize drug dosing, and perhaps serve as surrogate efficacy measures. Drugs that enhance central cholinergic neurotransmission have been extensively examined for their effects on regional cerebral blood flow, regional cerebral glucose metabolism, or levels of N-acetyl-aspartate in AD patients. Putative disease-modifying agents will be studied by structural magnetic resonance imaging for their ability to reduce rates of hippocampal or whole brain atrophy. When anti-amyloid therapies reach widespread clinical testing, their effects may be monitored by specific imaging probes for amyloid-beta peptide-containing senile plaques. Neuroimaging studies of AD patients and asymptomatic subjects who carry the apolipoprotein E e4 allele suggest that these individuals may be particularly suitable for testing candidate prevention therapies for AD.