Jay N. Giedd, M.D. is a practicing Child Psychiatrist, adjunct Professor at John Hopkins Bloomberg School of Public Health (Dept. of Family and Reproductive Medicine) and Chief of the Brain Imaging Section at the Child Psychiatry Branch of the National Institute of Mental Health. Since 1991, in the lab of Dr. Judith Rapoport, he has been conducting a large scale longitudinal study combining brain imaging, genetics, and psychological/behavioral assessments to explore the path, mechanisms, and influences on brain development in health and illness. The data set comprises 9000+ scans from 3500+ subjects – ¼ typically developing singletons, ¼ typically developing twins, and ½ from 25 clinical populations (most prominently Autism, ADHD, and Childhood-onset Schizophrenia). The study has led to over 200 papers covering a wide range of topics and has helped advance neurodevelopmental hypotheses of psychiatric disorders that remain the focus of several ongoing investigations throughout the world. The studies of typical development yielded novel insights as to why so many psychiatric disorders emerge during adolescence and characterization of the neurobiology of the teen brain has had a prominent impact in the realms of education, neuroscience, the judicial system, and public policy. Data from the earliest longitudinal pediatric neuroimaging twin study is elucidating Age * Gene * Environment interactions that have important implications for the design and interpretation of imaging genetic studies across all ages. Studies of people with atypical numbers of X or Y chromosomes (e.g. XO, XXX, XXY, XYY, XXXY, XXYY, XXXXX, and XXXY; total N = 300) are beginning to shed light on why all neuropsychiatric disorders of childhood have different ages of onset, prevalence, and symptomatology between males and females. The current focus is on application of the brain development insights obtained to guide interventions and improve the lives of youth and their families.