The design and manufacture of a variety of types of boron-trifluoride proportional counter used by the U.K.A.E.A. are described. One basic design and method of assembly is used for the manufacture of counters of sizes ranging from ½ in diameter and a few inches long to 2 in diameter and several feet long. The technique of manufacture is suitable for large-scale production, and consistently good electrical characteristics can be obtained. Details are given of the operating characteristics of a wide range of counters, and the effect of circuit parameters on the output pulse amplitude is discussed. Measurements of the γ-response of medium-size counters show that they can be used to discriminate between neutrons and γ-radiation at γ dose-rates of 100-200 rads/h. The counters can be used for long periods at temperatures up to 100°C without significant deterioration in characteristics. The counting life is found to be a function of counter size and of gas multiplication, and is probably limited by dissociation of BF3 and the formation of electron-capturing dissociation products. A typical medium-size counter, e.g. 1 in diameter by 6 in long, can be used for 1011 counts at a gas-multiplication factor, M, of 10, before significant deterioration of the counting characteristics occurs. Some applications of BF3 proportional counters, such as in reactor instrumentation, and the use of large counter arrays in nuclear physics experiments, are discussed briefly.