Tics are sudden, quick, repetitive, stereotyped, "relatively involuntary" muscle contractions that can occur in any part of the body. Tic disorders are highly prevalent in children and adolescents but cause severe impairment in only a small minority. However, tic disorders also are model neuropsychiatric conditions that provide a unique window into the interplay of genetic risk, psychology, experience, and environment. Exploring tic disorders has led to a deeper understanding of neural pathways and circuits in the brain that subserve sensory and motor function, linking the frontal lobes, the striatum, and thalamus. The study of tic disorders has increased our knowledge of the relationship between cognition and motor activity and illuminated the role of striatum in motor planning and execution. Treatments now include interventions that draw directly on this preclinical and clinical work.