One cannot read the burgeoning literature on TS and other neuropsychiatric disorders without being aware that comorbidity as a concept has been receiving increasing interest, 6,47,61,62 and rightly so. Nevertheless, although there is new respect for this concept, there is also considerable confusion over its theoretical and practical implications, among both professionals and the general public. Perhaps nowhere else is this more obvious than with the combination of TS and attention deficit hyperactivity disorder (ADHD). This is exemplified by the well-known observation that in less than 20 years we have gone from a fear of causing or permanently exacerbating TS by the use of stimulant drugs, to their increasing usage. Because the literature is so extensive, this article focuses on only two major issues: First, does the undoubtedly common co-occurrence of the two conditions in our clinics and practices really indicate comorbidity? If it does, then there are important practical consequences for counseling patients and families and for teaching our students, as well as for the study of categorical boundaries. If it does not, then there are quite different consequences. This author chooses to refer to "co-occurrence" rather than "comorbidity" in most instances, because it is my opinion that the question is still open. Second to be considered is the present state of knowledge of how to manage the co-occurrence of TS and ADHD (regardless of what may presently be concluded about the first question), using a selective review of the literature.