In Alzheimer's disease (AD) patients, depression is not rare. The prevalence of major depressive episodes has been reported to be within the range of 20-25% in AD patients, despite there being no association between the severity of AD and prevalence of comorbid depressive symptoms or diagnosed depression. Depression in AD patients is associated with greater impairment of the quality of life and an increased caregiver burden. As well as earlier placement in a nursing home, bio-psycho-social factors are also associated with the manifestation of depression in AD patients, and biological factors, such as the brain pathology, may be the main influence. Depressive mood, loss of interest, and anxiety are among the most marked symptoms of depression in AD patients. In comparison with major depressive disorder, in depression in AD, psychomotor retardation is more prominent, while, in major depressive disorder, somatic anxiety is more marked. In the treatment of depression in AD, non-pharmacological and pharmacological therapies are applied. Basically, support and encouragement are required. In addition, psychosocial interventions, such as validation, reminiscence, physical exercise, and interventions for caregivers of those with dementia have been reported to be useful. The results of RCT with antidepressants are inconsistent. As the efficacy of cholinesterase inhibitor for depression in AD has been reported, it is reasonable to initially provide treatment with cholinesterase inhibitors rather than antidepressant therapy.