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Since the discovery of the X-ray effect in ionisation gauges in the late 1940s researchers attempted to decrease the lower limit of vacuum measurement . The intensity of this work varied with the interest of the possible users where the latest peak of interest had been in the early 1990s. Today, mostly the accelerator community seems to have interest in the measurement of extreme high vacuum.
This talk will review the physical and technological problems significant for the measurement of extreme high vacuum and the attempts made to their solution. The kind of solutions will include the developments for the so called cold and hot cathode ionization gauges as well as optical methods. The approaches to calibrate these gauges will be described as well.
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