Cold-cathode ion gauges are now used in a wide range of high vacuum applications. Because of their freedom from many of the errors associated with hot-cathode gauges, they are becoming increasingly popular for use at ultra-high vacuum. Their calibration at low pressures, especially below about 10@super -9@ Torr, presents special challenges because of the rather complex (yet stable) logarithmic nature of their current-pressure response. These parameters must be established by tests at known pressures extending to the 10@super -11@ Torr range or lower. This in turn requires the use of special hot-cathode reference gauges such as the extractor, modulated Bayard-Alpert or X-ray-neutralized Bayard-Alpert types. We describe calibration procedures and results for a number of cold-cathode gauges at pressures in the 10@super -8@ to 10@super -11@ Torr ranges. Some aspects of conventional calibration techniques may be inappropriate or counterproductive at very low pressures because of X-ray, outgassing and ion desorption errors in the hot-cathode reference gauges. Preliminary results with a newly-developed cold-cathode reference gauge are discussed.