Lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics. The methods of investigation include classical four-electrode resistivity depth probing, which is of special value in continental shields; magnetotellurics, long used on land and recently in the Pacific to show that in electrical terms the lithosphere is thicker, near an oceanic ridge, than had been expected; and magnetometer arrays, which are useful primarily in mapping highly conductive structures. Examples of the use of all three methods will be reviewed, with emphasis on the last. In conjunction with other geophysical, geological, and geochemical data, magnetometer arrays have been useful in mapping partial melt under the western United States, in discovering an extension of the southern end of the African rift system to the Atlantic coast, in detecting a major fracture zone in central North America which may be a Proterozoic interplate collision feature, and in discovering a conductive belt under southernmost Africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under Proterozoic Gondwanaland. Inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two- and three-dimensional conductive structures. This remains valuable, however, particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures. Recent array studies now in progress in western Canada have defined two new conductivity anomalies. One of these may represent a local partial melt concentration under the Rocky Mountains Trench, the other a Precambrian rift valley in the lower crust previously suggested by others on the basis of seismological, gravity, and static magnetic data. Both are being investigated by more concentrated arrays.