[Clin Psychol Sci Prac 17: 265–271, 2010]
As an introduction to the Special Issue that includes a series of articles on comorbid mental health conditions among youth, some issues pertinent to the diagnostic system are considered. The discussion of illustrative issues that affect the diagnoses of mental health problems among youth includes reminders to consider the source, the time frame, what’s normal, the situation, the data, and the effect of changing diagnostic criteria. We support the DSM and ICD efforts to continue to develop as instruments by relying on the data.