Depression and dementia, in particular Alzheimer's disease (AD), are critically important issues in the mental health of old age. Both conditions apparently reduce quality of life and increase the impairment of activities of daily living for elderly persons. AD usually shows poor prognosis owing to progressive neuronal degeneration, while depression is basically reversible. However, depressive symptoms are common in AD and occur in approximately 20-30% of patients with AD. Epidemiological studies have shown a possible pathological association between depression and AD. Some longitudinal studies have reported that depression is a prodromal sign or might be both a prodromal symptom of AD and a risk factor. Other studies have suggested that depressive symptoms appear to coincide with or follow the onset of AD rather than precede it. However, it still remains controversial whether depressive symptoms represent a risk factor for AD, whether they are an early symptom of neurodegeneration, or whether they are a reaction to early cognitive deficits. A better understanding of the link between AD and depression might have important clinical and research implications. This review provides an overview of current knowledge regarding a relation between depression and AD and also proposes a research and clinical perspective on depression in AD.