The measurement of pressure in the UHV range started in 1950 with the invention of the Bayard-Alpert gauge which reduced the lowest pressure measurable with hot-cathode gauges of the time by a factor of 200 or more. The processes causing limits to the lowest measurable pressure include the x-ray effect, electron stimulated desorption from electron bombarded electrodes, chemical effects at thermionic cathodes, and thermal desorption. This historical review traces from 1950 to the present day the development of gauges, both hot and cold cathode, designed to reduce these lower limits, and emphasizes the research directed to understanding the physical and chemical processes governing these limitations. The development of, and the limiting processes in, residual gas analysers suitable for UHV is also followed over the same period starting with the omegatron, then the magnetic sector instrument, and finally the quadrupole.