This thesis is comprised of five genetic studies on Tourette Syndrome (TS) and its commonly co-occurring psychiatric disorder, obsessive-compulsive disorder (OCD). The main goals of these studies are to understand the genetic architecture of TS and OCD, to identify the underlying genetic variants, genes, and genetic pathways, and to investigate the genetic relationship between TS and OCD. The studies in this thesis demonstrate how analyses of common variants form our current understanding of the genetics of TS and OCD, and how these findings would inform future genetic studies of these two disorders.