As solar activity moves toward an 11 year peak, utility engineers are girding for the effects of massive magnetic disturbances. The nature of the geomagnetic disturbances is examined. The way in which geomagnetically induced current (GIC) affects power systems is explained. Virtually all power equipment, operation, and protection problems due to GIC are traceable to two direct effects: the half-cycle saturation of power transformers and the half-cycle saturation of the current transformers used with protective relay systems. Of the two, the former, with its numerous secondary effects, has been the more serious. As an example, a blackout due to solar disturbances, which happened to the Hydro-Quebec system in Mark 1989, is described. The factors that make power networks especially vulnerable today and the difficulty of predicting episodes are considered.