The research literature on mood disorders in children and adolescents has grown considerably over the past 20 years, following empirical demonstrations that children and adolescents can present with classic signs of mood disorders as evinced by adults (Carlson & Cantwell, 1980), and that depressive disorders identified in adulthood often onset at a relatively young age (Kessler, Avenevoli, & Merikangas, 2001). Although developmental psychopathology research on depression has lagged behind that on externalizing disorders, considerable data regarding the phenomenology, course, and correlates of mood disorders in children and adolescents has accumulated. This literature has pointed to a number of issues and challenges related to the conceptualization and assessment of depression across the lifespan. In this chapter, we review key issues in the measurement of depression in children and adolescents as they relate to phenomenology, etiology, course, and treatment outcome. Existing data indicate that the prevalence of frank mood disorders exhibits a strong developmental trend, with very low rates in preschool-aged children (Kashani & Carlson, 1987), increasing slightly in the elementary school age (Fleming & Offord, 1990; Cohen et al., 1993), and rising to levels similar to that among adults in adolescents (Kessler et al., 1994; Lewinsohn, Rohde, & Seeley, 1998). Therefore, researchers and clinicians working with depressed children and adolescents must keep in mind that the base rates of threshold levels of these disorders are quite low. Nonetheless, it appears that early onset mood disorders and depressive symptoms in childhood and adolescence are linked to a number of indicators of poor outcome, including later psychopathology and impairment in important life domains (Harrington, Fudge, Rutter, Pickles, & Hill, 1990; Luby, Todd & Geller, 1996; Birmaher et al., 1996; Rudolph, Hammen & Burge, 1994). Therefore, depressive disorders and symptoms present an important assessment issue for clinical and research work with children and adolescents, and the assessment of childhood onset of these disorders is relevant to understanding course and outcome among adult samples.