Primary care providers are in the front line of detecting and diagnosing psychiatric illness. Managed care barriers to direct psychiatric treatment have made it necessary for primary care providers to increase their sophistication in the recognition of psychiatric disorders. Primary care providers often formulate provisional diagnoses and initiate treatment or specialty referral in spite of the time constraints of the primary care setting. The patient presenting in primary care with an affective disturbance must be evaluated for a major mood disorder, which includes unipolar and bipolar illness. Research has shown that more patients than previously estimated have milder forms of bipolar illness disorder, such as bipolar type II and cyclothymia. Patients with these milder forms of bipolar are less likely to present for treatment in a psychiatric setting and more likely to share symptoms of the illness in a primary care setting. This article provides an overview for the primary care provider in the detection, assessment, and treatment of bipolar patients with an emphasis on the differentiation of unipolar and bipolar depression.