To investigate the reliability and validity of the Yale Global Tic Severity Scale (YGTSS), 28 youth aged 6 to 17 years with Tourette's syndrome (TS) participated in the study. Data included clinician reports of tics and obsessive-compulsive disorder (OCD) severity, parent reports of tics, internalizing and externalizing problems, and child reports of depression and anxiety. All children participated in a 2nd YGTSS administration by the same rater 48 days later. Good internal consistency and stability were found for the YGTSS scores. YGTSS scores demonstrated strong correlations with parent-rated tic severity (r = .58-.68). YGTSS scores were not significantly related to measures of clinician ratings of OCD severity (r = .01-.15), parent ratings of externalizing and internalizing behavior (r = -.07-.20), and child ratings of depression (r = .02-.26) and anxiety (r = -.06 -.28). Findings suggest that the YGTSS is a reliable and valid instrument for the assessment of pediatric TS.