In addition to a sample of 200 ADHD families (200 probands, 400 parents, and 150 siblings) being recruited in a parallel study (NSC96-2628-B-002-069 -MY3), we will recruit another 100 probands with ADHD, aged 7-18, and their parents (n = 200) and siblings (n= 75) in this project. The phenotype measures include (1) interviews for psychopathology (K-SADS-E) and social functioning (SAICA), (2) self-administered questionnaires to measures ADHD symptoms (SNAP-IV and Adult ADHD rating scale) and comorbid conditions (ASRI and CBCL), and (3) Neuropsychological tests: WISC-III, CPT, CANTAB, and Time Perception Tasks. The DNA will be collected and analyzed. The transmission/disequilibrium test (TDT) and quantitative TDT will be used in data analysis.