In the past 32 years, lasers have progressed from interesting curiosities to important instruments in medicine and surgery, with a wide variety of wavelengths and medical-surgical applications. Today's physician is faced with a bewildering array of laser types, each touted by its manufacturer as the ultimate surgical tool. This article sets forth in simple, understandable prose, the basic principles of the interaction of laser light with living tissue, so that the physician can decide which type of laser is best suited to a given application, without relying on the manufacturer's sales literature. The topics discussed are the nature of light; reflection, absorption, scattering, and attenuation in living tissue; physical processes by which laser light destroys tissue; relative importance of the three unique properties of laser light in surgery and therapy; temporal modes of lasers; means of delivering laser beams to their targets; and considerations in the selection of laser systems.