This paper describes the evolution of different concepts of classifying and defining Temporomandibular Disorders (TMD) for both clinical and research settings. The literature is reviewed with respect to the utility and validity of the different questionnaire and examination procedures that have been used to assess TMD patients. The presented view is that many of these procedures have not been validated, that there is a lack of standardization in the use of the procedures themselves, and that an ideal method of classifying this broad group of patients into better-defined subgroups has not yet been developed. More standardized and better-defined research by trained and calibrated researchers is needed worldwide to elucidate these subgroups so that a better and widely agreed upon research classification system can be developed for widespread use. It also seems clear that as research requirements for defining TMD patient subgroups become more stringent over time, it may not be practical for the clinician to implement them on a day-to-day basis in his or her practice. As such, a practical utilitarian definition of the common subtype of TMD patients is also needed which parallels any research grouping, so that data from research are valuable and generalizable to the practicing clinician.