This paper reviews the use of electrical measurements of caries, particularly in relation to caries clinical trials. Electrical measurements change as tooth tissue porosity alters in the caries process, but several other variables also have a significant effect on these electrical measurements and hence upon their diagnostic validity. Available electrical-method data, in the context of clinical trials, relate to the use of the Electronic Caries Monitor (ECM), which measures "bulk" resistance. The device is presently limited in scope to occlusal surfaces, and only limited ECM data from clinical trials are available. In the context of clinical trials, more work is needed to determine the potential role of electrical measurements. Such research will need to focus both on an understanding of those electrical parameters which are most valuable in identifying changes and stages in the caries process in individual teeth and also on identifying the extent of the effects of the variables affecting these measurements.