Sections 2–6 give a summary of the available knowledge of surge phenomena and for convenience of treatment this is divided into five parts; lightning and lightning surges, surge propagation, distribution of surge voltages in inductive apparatus, effects of surges on insulation and surge protective devices.The first part of this indicates the importance of lightning-current measurements and specifies the various types of lightning surge which can occur on overhead lines. The second part draws attention to those aspects of surge propagation which are important in relation to the protection of substation apparatus, with special emphasis on the protection afforded by attenuation by corona. The third part discusses the problem of designing transformers to withstand surges and refers to the question of surge testing. The fourth part gives a summary of the available data on the surge strength of relief gaps, line insulators and cables. The fifth part summarizes the characteristics of recognized protective devices.The summary of available knowledge is followed by specific recommendations for its application to the protection of various types of overhead transmission line and of substation.Finally, attention is drawn to the need for further research, and suggestions for new researches or extensions of existing researches are made.