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Cerenkov counters as particle detectors in high energy physics experiments are discussed with emphasis on the practical design of velocity-sensitive devices. The performance and problems associated with three types of detectors are considered: simple velocity threshold counters and wide-band and narrow-band velocity selectors. The limitation in resolution of practical velocity-sensitive counters in high energy experiments arises mainly from the characteristics of the beams which must pass through their radiators. These limitations include divergences in the direction of the beam particles, multiple coulomb scattering, and changes in velocity of particles as they pass through the Cerenkov radiator. Methods of coupling radiators to multiplier phototubes include direct optical contact, specular reflection, and diffuse reflection. Magnesium oxide is an excellent diffuse reflector and methods of its application are given. Statistical fluctuations in the small numbers of photoelectrons produced from Cerenkov radiators limit the accuracy with which the times of passage of individual particles are determined.