The technology of ultra-high vacuum (U.H.V.) was established in the two decades between 1950 and 1970. Although there have been no recent developments of major significance the basic principles of U.H.V. are being used in applied science and engineering application on an increasing scale. There are large machines at CERN which depend entirely upon U.H.V for their operation. Similar dependence is seen in thermonuclear research, for example in the Joint European Torus. In the field of semiconductor device manufacture the techniques of molecular beam epitaxy require the very best vacuum conditions. This review concentrates upon the problems of monitoring both the total and partial pressure of gases at very low pressures from 10−8mb to below 10−12mb. Emphasis is upon methods which are in established practical use. All are based upon the simple ionization gauge as modified by Bayard and Alpert in their pioneering work. For residual gas analysis, interest is divided between the magnetic deflection and the quadrupole mass spectrometers.