Ionization gauge and method of operating same where the gauge may be of the Bayard-Alpert type and include a shield which completely encloses the electron source, the anode, and the collector electrode so that potentials external to the shield do not disturb the electric charge distribution within the shielded volume to thus stabilize the sensitivity of the gauge. The ionization gauge is further characterized by the following features which may be present either alone or in combination including: (a) the anode is provided with end caps which extend radially inward at least 25% of the radius of the anode but not more than 75% of the radius; (b) the ion collector has a diameter of not less than 0.015 in. and not more than 0.080 in.; (c) the ion collector extends at its free end at least through one of the partial end caps of the anode; (d) the electron source is not substantially longer axially than the region within the anode volume in which the electric field is predominantly directed radially inward and where the electron source is located substantially only adjacent to the foregoing region; (e) the anode is provided with a substantially uniformly transparent grid structure in the regions where the electrons enter the anode volume and where they exit from the anode volume.