Biological markers are important tools in identifying predisposing factors to disease, as diagnostic tests, and in monitoring disease progression. Alzheimer disease is believed to have a long preclinical phase, followed by mild cognitive impairment, and, finally, dementia. Detecting alterable predisposing factors or identifying patients in preclinical or early-stage illness offers the greatest potential to modifying disease course. The authors focus on: 1) predisposing factors, such as genetic risk factors and homocysteine; 2) laboratory markers, such as amyloid beta and tau protein; and 3) diagnostic markers, such as structural and functional neuroimaging. Many markers have been tested but have not been confirmed in subsequent studies. Other tests require complex and expensive laboratory evaluation or expertise, thereby limiting their use at present. Still others are markers only useful in later-stage illness. Nonetheless, the search for markers has increased our understanding of the biology of illness and has led to exciting new directions, with diagnostic as well as treatment implications.