The increasing sophistication of modern technology and its widening areas of application have established the need for multidisciplinary activities, and biomedical engineering is a typical example of this trend. The evolution of the subject from the engineering approach of earlier biomedical workers is described, and some of the lessons learned before and during the transition are considered. An approach to this now rather extensive subject is made by separating the application to biomedical research from that to clinical medicine, The research section is stratified according to the level of biological organisation with which it is concerned, that is, from the molecular systems of biochemistry to the organ systems of physiology. At each level, sufficient background is given to show the relevance of biomedical engineering, and this approach is continued with the clinical applications, which are considered within conventional medical categories. The principal contribution to medicine is found to lie with the innovation of the new methods made possible by advancing technology. The more recent technological ideas of organ and function replacement are discussed followed by that of computer usage in various areas of biomedicine. In conclusion, a critical assessment of present achievements is attempted, and the possible future of biomedical engineering is discussed.