A critical step for the future of the profession of physical therapy is the development of diagnostic categories. The purpose of this communication is to clarify issues regarding the role of the physical therapist in developing classifications of the signs and symptoms that are identified by the therapist's examinations and tests. A primary premise is that treatment should be based on the diagnosis derived by the physical therapist because the medical diagnosis does not provide sufficient direction. A generic definition is given as a guide for development of diagnostic classification schemes. Arguments are provided that these diagnoses will 1) clarify practice, 2) provide an important means of communication with colleagues and consumers, 3) classify and group conditions that can direct research and assessment of treatment effectiveness, and 4) reduce the tendency toward cultism associated with practice based almost entirely on treatment approaches.