From the inauspicious beginning arising in the aftermath of a reactor accident in 1957, in vivo analysis of body elements by neutron activation has become an important tool in medical research. In particular, it provides a much needed means to make quantitative assessments of body composition of human beings in vivo. The data are useful both for basic physiological understanding and for diagnosis and management of a variety of diseases and disorders. The paper traces the development of the in vivo neutron activation technique in the past quarter of a century from basic systems to the present state of the art facilities, and discusses the advantages and disadvantages of the various systems. A scan of some of the numerous clinical applications that have been made of this technique reveals the broad potentialities of in vivo neutron activation. The paper also considers alternative routes of future development and raises some of the questions now faced in making the techniques more widely available to both medical practitioners and medical investigators.