The modern design of outdoor HV insulators is determined by rules which follow from characteristics of flashover mechanisms, such as: 1). The leader-streamer mechanism of breakdown of gaseous gaps close to dielectric surface under impulses of overvoltages, (determines necessary height of insulator, and also the form and sizes of its electrode system); 2). The mechanism of flashover of insulator on an AC by partial arcs with polluted and humidified surface, (determines number of sheds, its profile and size).