Depression is the psychiatric syndrome that has received the most attention in individuals with cancer. The study of depression has been a challenge because symptoms occur on a broad spectrum that ranges from sadness to major affective disorder and because mood change is often difficult to evaluate when a patient is confronted by repeated threats to life, is receiving cancer treatments, is fatigued, or is experiencing pain. Although many research groups have assessed depression in cancer patients since the 1960s, the reported prevalence (major depression, 0%-38%; depression spectrum syndromes, 0%-58%) varies significantly because of varying conceptualizations of depression, different criteria used to define depression, differences in methodological approaches to the measurement of depression, and different populations studied. Depression is highly associated with oropharyngeal (22%-57%), pancreatic (33%-50%), breast (1.5%-46%), and lung (11%-44%) cancers. A less high prevalence of depression is reported in patients with other cancers, such as colon (13%-25%), gynecological (12%-23%), and lymphoma (8%-19%). This report reviews the prevalence of depression in cancer patients throughout the course of cancer.