This article: (1) describes and reviews evidence for hypothesized biological and psychological mechanisms of winter seasonal affective disorder (SAD), (2) advocates for an integrative approach to studying SAD etiology that incorporates both biological and psychological mechanisms, and (3) delineates areas for future research from an integrative perspective. Exciting progress has been made within sub-paradigms testing candidate biological mechanisms (i.e., biological rhythm abnormalities, retinal subsensitivity to light, neurotransmitter alterations, and genetic variations) and psychological mechanisms (i.e., maladaptive cognitions and behaviors) of SAD. However, research from an integrative biological/ psychological perspective is currently lacking. In contrast to a continued exclusive focus on micro-models, we argue that an integrative approach would maximize the capacity to predict and understand the onset, maintenance, and course of SAD. An integrative approach also provides a comprehensive theoretical framework for developing strategies to effectively treat acute SAD, maintain acute treatment gains throughout the winter, and prevent future episodes of this highly recurrent form of depression.