In his treatment of activity measurement in the fields of medicine and psychology, Tryon gives us a book that clearly accomplishes the three purposes set out in its preface. The reader is definitely encouraged to wrestle with the concepts ofbehavior and activity in terms of "dynamic physical quantities." Moreover, the reader cannot help but become familiarized with the technology available for performing activity measurements. Motivation to use some of this technology is enhanced by the very extensive summary of other people's uses of it provided throughout the book. Readers may find the book provocative on a number of Ievels. It is concep­ tually provocative to those of us struggling with understanding basic issues in the assessment and measurement of behavior. It is practically provocative to those of us working with various forms of behavioral difference, especially in clinical popula­ tions. The book provokes because it is essentially an unfinished exploration, open­ ing us to numerous pathways that, when traveled, reveal still more paths to explore. In this sense the book should be heuristically useful both in the more traditional empirical sense, and in terms of its Stimulation of conceptual discussion.