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Surface flashover in compressed-gas insulated systems is a much studied, but poorly explained phenomenon. In this paper we review the literature of surface flashover with primary emphasis on the understanding of physical processes leading to discharge initiation and insulator flashover under high voltage excitation. The flashover models presently in vogue will first be discussed, followed by the results of some recent experiments which are likely to have an impact on further modeling. Included in this context are phenomena such as ionization, surface charging, partial discharges, optical activity, and gas/dielectric interactions. Finally, the influence of system parameters such as insulator size, shape, surface condition, triple junction geometry, voltage waveform, gas formulation and particle contamination are discussed with regard to their effect on the flashover characteristics. Mechanisms are suggested in an effort to provide a physical explanation for the observed phenomena. Although the physics of the discharge initiation and propagation processes are presently not well understood, and the present models only account for a few of the mechanisms known to be important in the discharge development, all the work points to an interaction between the spacer and the various electron/photon processes in the surrounding gas volume. This interaction has not been accounted for in the discharge models proposed to date. Further modeling work should incorporate these interactions and the intrinsic properties of the dielectric ric which are related to these interactions. More basic research is suggested to provide a better understanding of the physics of the discharge initiation and breakdown phenomena.