The Bayard Alpert (BA) ionization gauge is the most common device used for the measurement of pressure in vacuum systems. There are however many potential problems in the use of the gauge and in the interpretation of the data that it provides. Perhaps the most basic problem is that the sensitivity of the gauge is substantially different for the common gases encountered in a vacuum system, including hydrogen, argon and nitrogen: if the gas composition is unknown, the absolute pressure cannot be determined. Nevertheless, in many systems, where the gas composition remains constant from day to day, the reading from a BA gauge can serve as a meaningful indicator of relative pressure. But if something goes wrong in the system, such as a water leak, the gauge alone can provide virtually no information as to the problem. This paper is primarily directed to the many other problems that can afflict a gauge; it is an attempt to give practical guidance on the use of a gauge, such as the appropriate connection to a system, and the operating techniques which can be used in order to obtain meaningful data. The topics addressed include gas pumping, by generation of ions, and by chemical interactions at the gauge filament; the change in gas composition by interaction at the gauge filament, and errors in pressure measurement including Barkhausen-Kurtz oscillations, electron stimulated desorption, and the x-ray effect. Factors which dictate the specific BA gauge selection, such as the method used for outgas, and the selection of the electron emitter, will also be discussed.