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The technique of intrinsic safety was originated in England for mine signalling, following a serious Welsh mine disaster in 1913. The first surface certificate was issued in 1936 and the first formal standard in 1945, but it was not until the 1960s that the technique came into its own, with the advent of semiconductors. Now intrinsic safety has become the predominant solution in Europe for process measurement and control in hazardous locations, and is seriously challenging the flameproof approach in North America. This paper outlines the development of the technique, its application rules, and some typical applications of shunt-diode safety barriers and intrinsically safe “interface units”.