1. Inclusion of children, particularly healthy control children, is a continuing debate. 2. Why involve children in PET research? The assumption is that the knowledge gained from such studies is critical for the advance of prevention and treatment of psychiatric illnesses. 3. What are the risks of PET procedures? Radiation exposure poses the most difficult problem. The assessment of this risk needs to separate the emotional reaction at the mention of "radiation" from the consideration of objective data of large studies of health hazards associated with low-level radiation exposure. 4. The assessment of the benefit/risk ratio is critical to the conduct of research, and requires the evaluation of risks according to the ambiguous definition of "minimal risk".