Factors affecting the accuracy of ionization gauge measurements at low pressures are reviewed. In hot-cathode gauges these include electron-stimulated desorption at the electron collector, forward and reverse X-Ray effects, Auger emission, outgassing, and various controller-related errors. In cold-cathode gauges they include nonlinearities below the "magnetron knee", plasma instabilities, and leakage currents. Case studies are given to illustrate many of these sources of error and their elimination. The case studies were gathered in the course of long-term stability measurements on over 30 ionization gauges at pressures ranging from 10@super-7@ to 10@super-11@ Torr. The investigation included Bayard-Alpert (both conventional and modulated), Extractor, Magnetron, Inverted Magnetron and Double Inverted Magnetron gauges. Recent measurements on Bayard-Alpert gauges with low-temperature (lanthanum boride) and cold (disordered tetrahedral carbon) emitters are also discussed. It is concluded that, with proper precautions, ten percent reproducibility in the 10@super-10@ Torr range is easily achievable with either hot-cathode or cold-cathode gauges. A combination of the two, mounted on a common vacuum flange, is particularly useful at very low pressures.