This article presents an overview of the conceptual basis and empirical support for a unitary view of mood disturbance. The authors consider this general conceptualization of psychiatric disturbance as consisting of an array of related symptoms and behavioral features that define the "spectrum" of each disorder. They discuss how this conceptual framework aids in overcoming what they believe to be the false dichotomy between unipolar and bipolar mood disorders and in evaluating subthreshold and unusual presentations. They also describe the structured clinical interview and self-report instrument that they have developed to facilitate systematic assessment of the proposed mood spectrum. Finally, they summarize the clinical utility of such an approach to the description and assessment of patients with mood disorders.