This chapter summarizes the data suggesting that the liability to tourette syndrome (TS) and related disorders is heritable. It focuses on the apparent role of L-histidine decarboxylase (HDC) in a two-generation family with nine affected individuals. The chapter considers the results and future promise of non-parametric and other model-free methods of gene identification. It discusses the merits of cytogenetic methods to search for rare genetic variants based on chromosomal rearrangements, with particular attention to Slit and Trk-like family member 1 (SLITRK1). Finally, the chapter provides the future prospects for TS genetic research and close with a discussion of the clinical and research implications of the increasing understanding of the genetic underpinnings of this syndrome. Multiple lines of evidence indicate that TS is an inherited disorder. The first and most important piece of evidence is that TS and related disorders tend to aggregate across multiple generations within families.