The focus of this article is on internalizing problems that are experienced by children and adolescents. We provide an historical perspective, selectively examine the current state of knowledge, consider advances and gaps in what is known, and identify new research directions. Diagnosis, epidemiology, theory, and research first are considered separately for anxiety and depressive disorders. These internalizing problems, however, whether clinical or subclinical, share many common features and show high comorbidity rates. We emphasize the importance of systematic analysis of comorbid anxiety and depression, including their comorbidity with externalizing problems. This could lead to more valid classification of subtypes of internalizing problems and further an understanding of the diverse conditions that constitute internalized distress. We highlight the need to study anxiety and depression within a developmental psychopathology framework, as well as to include both categorical and dimensional assessments of these problems in the same research designs. This will be essential for understanding the complex interplay of biological and environmental processes that contribute to the emergence, progression, and amelioration of internalizing problems over time.