An analysis is made of various sorts of equipment breakdowns and damages induced by the electromagnetic pulses of thunders and lighting, thunders and lighting classification, and the invasion approaches of thunders and lightning into communication systems and computer networks. The types, components, and technical specifications of SPDs (Surge Protector Devices) are introduced. The emphases are put on the criterions for selecting SPDs for communication systems and computer networks. According to the detailed calculation of maximum continuous operating voltage and maximal discharge electric current of SPDs, the reasonable methods for selecting SPDs are presented based on the types of connection equipments and the demands of working voltage and frequency.