Recent years have evidenced a tremendous increase in research using a developmental psychopathology framework to examine clinical diagnoses among youth. Despite this increase, a relative dearth of literature systematically examines the development of co-occurring conditions among youth. In this introduction to the Special Issue on comorbidity among youth, we suggest that a developmental psychopathology perspective can provide an important foundation for the diagnosis of mental health problems among youth. As a potential framework for future investigations, we consider several developmental psychopathology principles that can inform assessment and diagnosis among youth psychological disorders. We use these principles as a foundation for considering co-occurring psychological disorders and provide potential explanations for comorbidity that can be addressed in future research that uses a developmental psychopathology perspective.