The major developments in the measurement of vacuum since the Bayard-Alpert gauge was invented in 1950 are reviewed; the American Vacuum Society was started three years later in 1953. These developments include (a) the improvement in understanding the processes causing limitations of the lowest measurable pressures (both total and partial), (b) the development of gauges, residual gas analyzers, and optical methods of pressure measurement to reduce these limitations, (c) the introduction of room-temperature electron sources (Spindt cathodes) to replace thermionic cathodes, and (d) the development of the spinning rotor gauge as a secondary standard. (C) 2003 American Vacuum Society.