The major developments in the measurement of vacuum since the Bayard-Alpert gauge was invented in 1950 are reviewed.These include a) improvements in understanding the processes limiting the lowest measurable pressure (both total and partial), b) development of new gauges, residual gas analysers,and optical methods to reduce these limitations, c) introduction of room temperature electron sources to replace thermionic cathodes,and d) development of the spinning rotor gauge as a secondary standard.