Several studies have demonstrated that the great majority of Alzheimer's disease (AD) patients suffer "noncognitive" neuropsychiatric symptoms. Depression is one of the most frequent neuropsychiatric comorbidities of AD. Affecting up to 50% of AD patients, depression in AD is associated with serious negative consequences for patients and their caregivers. Yet available studies on the natural course, etiology, and treatment of depression in AD have been few and equivocal. Heterogeneity in research methodology and etiology of depression in AD might have contributed to inconsistent findings across studies. Recently, the National Institute of Mental Health (NIMH) convened the Depression of Alzheimer's Disease Workgroup, which proposed provisional diagnostic criteria for depression of Alzheimer's Disease (NIMH-dAD). These criteria may provide a framework for future studies to clarify the unresolved issues in nosology, etiology, and treatment of depression in AD. A longitudinal cohort study of depression in incident AD cases may provide further syndrome refinement that would facilitate investigation of the etiology and treatment of depression in AD.