Tourette's syndrome (TS) is a neuropsychiatric disorder characterized by a changing repertoire of motor and phonic tics that typically begin in childhood and exhibit a fluctuating course. Obsessive-compulsive symptoms and problems of inattention, overactivity, and impulsiveness are present in some cases. Historically, TS has been viewed as a severe disorder. However, data from epidemiologic and family-genetic studies indicate that it shows a wide range of severity. This article, the first of two parts, examines the current notions concerning the origin, pathophysiology, and diagnosis of TS. The second article will present three cases and describe contemporary approaches to treatment.