Gilles de la Tourette's syndrome (GTS) consists of multiple motor tics and one or more phonic tics. Psychopathology occurs in approximately 90% of GTS patients, with attention-deficit/hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD) being common. Depression is common, with a lifetime risk of 10% and a prevalence of between 1.8% and 8.9%. Depression and depressive symptoms are found to occur in 13% and 76% of GTS patients attending specialist clinics, respectively. In controlled studies embracing over 700 GTS patients, the patients were significantly more depressed than controls in all but one instance. In community and epidemiological studies, depression in GTS individuals was evident in two of five investigations. Clinical correlates of depression in people with GTS appear to be: tic severity and duration, the presence of echophenomena and coprophenomena, premonitory sensations, sleep disturbances, obsessive-compulsive behaviors/OCD, self-injurious behaviors, aggression, conduct disorder (CD) in childhood, and, possibly, ADHD. Depression in people with GTS has been shown to result in a lower quality of life, potentially leading to hospitalization and suicide. The etiology of depression appears to be multifactorial. Bipolar affective disorder (BAD) and GTS may be related in some individuals. However, it is noted that sample sizes in most of these studies were small, and it is unclear at the present time as to why BAD may be overrepresented among GTS patients.