As illustrated in Chap. 1, the wide utilisation of gas-fired generation and the rapid development of power-to-gas technologies have led to the intensified integration of electricity and gas systems. The random failures of components in either electricity or gas systems may have a considerable impact on the reliability of both systems. Therefore, it is necessary to evaluate the reliability of electricity and gas systems by considering their integration. In this chapter, a novel reliability evaluation method for integrated electricity-gas systems (IEGS) is proposed. First, reliability network equivalents are utilised to represent reliability models of gas-fired generating units, gas sources, power-to-gas facilities, and other conventional generating units in IEGS. A contingency management schema is then developed considering the coupling between electricity and gas systems based on an integrated electricity and gas optimal power flow technique. Finally, the time-sequential Monte Carlo simulation approach is used to model the chronological characteristics of the corresponding reliability network equivalents. The proposed method is capable of evaluating customers’ reliabilities in IEGS, which is illustrated on an integrated IEEE Reliability Test System and Belgium gas transmission system [26].