The diagnosis of Alzheimer disease (AD) is made first by clinical criteria to establish progressive dementia and second by extensive laboratory evaluation to exclude other possible causes of dementia. There are no widely accepted biochemical markers to establish the presence of the disease in living patients. Sensitivity, specificity, and cost still limit the widespread use of single photon emission computerized tomography and positron emission tomography in diagnosis of the disease. Several cerebrospinal fluid markers, including amyloid β-protein precursor, are being developed to aid diagnosis and to use as surrogate markers for use during trials of therapy. Chromosomes 21, 14, and 19 genetic markers have recently been associated with or linked to AD. In the coming years, early identification of susceptibility or disease-causing genes may facilitate risk-factor modification or drug therapy to halt or at least delay disease onset. To date, therapy of AD has been limited to treating secondary behavioral symptoms. Recent studies of tacrine have shown significant improvement in cognition of treated patients, as demonstrated by neuropsycho-logic testing and global clinical measures. Several other approaches to treating AD are being developed. In the future, subgroups of AD may be identified by genetic markers that will each respond to specific modes of treatment.