In the Proceedings of the recent IAEA-WHO Seminar on Medical Radioisotope Scanning (IAEA 1959) the considerable amount of work is surveyed which has been carried out on the design of scintillation counter collimator systems for studying the distribution of gamma ray emitting isotopes within the body. A large part of this work has been concerned with the problem of arriving at a satisfactory compromise between the conflicting requirements for maximum resolving power and maximum sensitivity. Although isotope scanning techniques have many actual and potential clinical applications, the clinical tests which are most widely carried out at present and which involve in-vivo counting are those where the object of the measurement is to determine the amount of radioactive material in a given organ or tissue as a function of time. In applications of this type, the primary function of the collimator is to reduce the interference from radioactivity in tissues other than the one under study. The linear resolution required is dictated by the anatomical and physiologic factors which apply in each particular case rather than the need for resolving power. At the same time, if the patient is to receive the minimum possible radiation dose, the requirement for maximum sensitivity remains.Up to the present, scintillation counters embodying NaI-Tl crystals, about 2.5 em diameter and 2.5 cm depth, and fitted with lead collimators having a cylindrical aperture about 2.5 cm diameter and 5 to 7.5 ern depth (see Fig. la),