This document describes principle of insulation coordination, especially for lightning overvoltage. Lightning is a major stress on equipment and is a decisive factor in ensuring insulation coordination. Insulation coordination is a discipline aiming at achieving the best possible technic compromise for protection of persons and equipment against overvoltages, where caused by the network or lightning, occurring on electrical installations. It helps ensure a high degree of availability of electrical power. The ultimate goal of insulation coordination is to find the right balance between equipment reliability from a dielectric standpoint on the one hand, and their sizing and cost. Before we plan to design insulation coordination, it is very important to know a variety of overvoltages. That means each overvoltage has different characteristics. Understanding all the aspects of overvoltages are also required to select an arrester for insulation coordination. This document sums up modeling considerations for fast-front overvoltage studies generated by lightning. We can find the most appropriate model by following the rules described on chapter3. Calculating of lightning overvoltage requires simulation of the electrical equipment like overhead transmission lines, cables, arresters, towers and substations. To analyze wave propagation process due to lightning phenomena, EMTP-ATP software has been used, which is one of the widely used software tool for simulating electrical transients in power system. EMTP is used for switching and lightning surge analysis including insulation coordination, protective relay modeling and so on. By means of such simulation model, it is possible to investigate influence of many phenomena. Based on the results of simulations, we will appropriately take a measure.