Recent evidence has suggested that the commonly observed comorbid behavioral disorders of Tourette syndrome, including attention deficit hyperactivity disorder and obsessive-compulsive disorder clearly have an impact on cognitive, educational, and psychosocial function. These behavioral features of Tourette syndrome can be more debilitating than the cardinal motor features of the disorder and they require careful clinical assessment so that appropriate treatment intervention can be offered to patients and their families. This article focuses on objective neuropsychological and behavioral assessment of obsessive-compulsive disorder and attention deficit hyperactivity disorder which have proven useful for both the clinical evaluation and treatment of obsessive-compulsive disorder and attention deficit hyperactivity disorder associated with Tourette syndrome and in the research setting. Instruments with proven psychometric reliability and validity for assessing obsessive-compulsive disorder and attention deficit hyperactivity disorder which also have been useful in Tourette syndrome populations are discussed. Objective and quantitative assessment of these comorbid behavioral conditions greatly enhance our ability to treat the full neurobehavioral spectrum of Tourette syndrome.