The purpose of this study is to learn more about Obsessive-compulsive Disorder (OCD) in children. OCD usually has a slow onset, and symptoms that may remain at a stable level over time. A subset of children with OCD has a sudden onset and symptoms that fluctuate in severity over time. This study will also compare healthy children to those with OCD. This is an observational study; children who participate will not receive any new or experimental therapies.