Atopic dermatitis (AD) is an inflammatory skin disorder characterized by itching, chronically relapsing course, typical stribution, and family history of allergies. The pathogenesis of AD is unknown but the development of AD is undoubtedly influenced by immunologic abnormality. But the broad clinical spectrum of this disease cannot be explained simply by allergy or immunological abnormalities. In recent studios revealed skin barrier dysfunctions in AD. We consider immunological abnormality and skin harrier dysfunction as two core defects of AD. The wide range of clinical manifestation of AD can be more easily comprehended by partial overlapping of these two core abnormalities. These two core abnormalities influences each other. This sequence results in a 'vicious cycle', which plays the most important role in pathogenesis of AD.