Depressive disorders in childhood and adolescence are characterized by core persistent and pervasive sadness, anhedonia, boredom or irritability that is functionally impairing, and relatively unresponsive to usual experiences that might usually bring relief, such as pleasurable activities and interactions and attention from other people. The single most important distinction between depression as an illness and the “normal ups and downs” of childhood and adolescence is that depression is associated with functional impairment, mediated through the intensity, duration, and lack of responsiveness of depressed mood and associated symptoms.

Depressive disorders exist on a continuum, and are classified on the basis of severity, pervasiveness, and presence or absence of mania (1). At the mildest end of the spectrum are adjustment disorders with depressed mood, which are mild, self-limited, and occur in response to a clear stressor. Depression not otherwise specified (NOS), also referred to as “minor” or subsyndromal depression, is diagnosed in the presence of depressed mood, anhedonia, or irritability, and up to three symptoms of major depression (2). Dysthymic disorder is a chronic condition with fewer symptoms than major depression, but lasts a minimum of one year. Major depression is the most severe condition, with either sad or irritable mood, or anhedonia, along with at least five other symptoms, such as social withdrawal, worthlessness, guilt, suicidal thoughts or behavior, sleep increase or decrease, decreased motivation and/or concentration, and increased or decreased appetite. Minor depression and dysthymic disorder are functionally impairing and are often precursors to major depression (2,3). Furthermore, dysthymia and major depression can coexist in a state referred to as “double depression,” which is associated with a particularly chronic course (3). Rarely, young patients with major depression also have psychotic symptoms such as auditory hallucinations or delusions, usually with self-derogatory, paranoid, or depressive content.

Comorbidity is the rule rather than the exception in depressed children and adolescents, especially in clinical samples (4). Anxiety is frequently a precursor of mood disorder and may also occur simultaneously with depression. ADHD and depression are also often comorbid and the two disorders may be cotransmitted in families (5). Alcohol, drug, and tobacco abuse are associated with depression, and longitudinal studies suggest a bidirectional causality, with substance abuse both leading to, and occurring as a consequence of, depression (6,7,8). Conduct disorder is frequently comorbid with depression, particularly in prepubertal samples (9). Comorbidity with depression may arise because sharing of risk factors that are common to both conditions. For example, the comorbidity of mood disorders and behavioral disorders and substance abuse may be due to common factors, such as parental substance abuse and criminality, exposure to family violence, and family discord (10). Comorbidity may also arise because a condition is either a precursor or a consequence to depression, in the cases of anxiety and tobacco use, respectively.