The development of scintillation counting in the last few years has depended upon the substitution of a photomultiplier tube for the human eye in the detection of small flashes of light by luminescent materials traversed by radiations. A general description is given of the main types of photomultipliers and luminescent materials suitable for such work. The crystals most used in practice are thallium-activated halides or organic crystals such as anthracene or naphthalene. To a first approximation the amplitude of the light pulse produced by the absorption of high energy particles is proportional to the energy absorbed, so that a scintillation detector is, in essence, a proportional counter. Deviations from this linearity, however, occur particularly with organic luminophors.A short outline is given of the application of scintillation counting techniques to medical problems, to which they are particularly applicable owing to the very high sensitivity and small physical size of the detector. A description is given of needle scintillation counters for the study of the radioactivity of biopsy specimens; of 4 π counters using a liquid scintillating medium for the study of the radioactivity of samples of blood; and of the application of scintillation counters in the study of the radioactivity of normal and pathological brain tissue. The paper next describes new dosemeters developed by Dr. E H Belcher depending upon the measurement of total luminosity of the crystals exposed to ionizing radiations. They are shown to be sensitive, stable, reasonably independent of wavelength errors, and are an important development in clinical radiation dosimetry.The paper ends with a description of a scanning technique by which it has been possible to form photographic and visual images of the distribution of radioactive materials. In particular, an equipment is described embodying a "storage tube" used in conjunction with television scanning techniques.