The Bayard-Alpert (BA) ionization gauge is the most widely used gauge for vacuum pressure measurement above 10-7 Pa. Its use is ubiquitous, although it does have several significant limitations - even at modest pressures. For pressures below 10-7 Pa, several critical processes which are both physical and chemical in origin, limit pressure measurement using BA gauges. Many variations have been designed to minimize limiting factors, with varying degrees of success. In this talk, I will overview the approaches to vacuum gauging and gauge designs for pressure measurement in the UHV and XHV regimes, with a focus on the limiting mechanisms, gauge interactions with residual gases, and the strategies that have been proposed to overcome them.