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