Prevention of Alzheimer disease (AD) is a national and global imperative. Therapy is optimally initiated when individuals are asymptomatic or exhibit mild cognitive impairment (MCI). Development of therapeutically beneficial compounds requires the creation of clinical trial methodologies for primary and secondary prevention. Populations in primary prevention trials selected only on the basis of age will have low rates of emergent MCI or AD. Epidemiologically based risk factors or biomarkers can be used to enrich trials and increase the likelihood of disease occurrence during the trial. Enrichment strategies for clinical trials with MCI include use of biomarkers such as amyloid imaging, MRI with demonstration of medial temporal lobe atrophy, bilateral parietal hypometabolism on PET, and reduced amyloid beta peptide and increased tau protein in CSF. Neuropsychological measures appropriate for trials of MCI may not be identical to those measures most suited for AD trials. Attention to these and other features of trial design, clinical assessment, and use of biomarkers is critical to improving the detection of disease-modifying effects of emerging therapies in presymptomatic or minimally symptomatic populations. The neurologic health of the growing aging population demands disease-modifying therapies and the development of methods to identify and test promising candidate agents.