Atopic dermatitis (AD) is the most common inflammatory skin disease in children, may persist into adulthood, and is difficult to treat. Recent data in both experimental models and patients have shed new light on the multifactorial etiology of this chronic disease. In this review, we summarize the pathomechanism of AD in the following three sections: (1) defects in skin barrier including the role of filaggrin; (2) the immunological response of patients including key roles of T cells, dendritic cells, innate lymphoid cells, mast cells, and eosinophils; and (3) environmental factors such as the role of skin microbiota including Staphylococcus aureus.