Protein turnover is a fundamental biological process in all living organisms. The study of protein turnover in human subjects, and in the body as a whole, is of relatively recent origin. In this review, I summarize briefly how this field of work has expanded over the past 25 years, with emphasis on conceptual problems and controversies, particularly those related to methods of measurement. We cannot be certain that our estimates are accurate because no method of verification exists, but progress will be made through successive approximations. Some of the applications are described in different physiological and pathological states such as growth and trauma, and possible directions for fruitful future research are indicated.