This chapter brings together the reliability models in Chap. 2 and both the multi-energy flexibilities in the transmission section and demand side, which bring both opportunities and challenges to the reliable operation of integrated energy systems (IES). For example, if the electricity supply is interrupted, the gas system can ramp up the gas supply to the gas-fired units using linepacks. By this means, the reliability of the electricity system at this moment can be improved, while the gas system’s capability of withstanding future risks may be undermined. Therefore, the operational reliability between different energy systems and time periods should be carefully balanced. This chapter proposes an operational reliability evaluation framework for the IES that considers flexibility from both the demand side and the transmission system. Firstly, the flexibilities of end-users and linepacks are explored based on the Energy Hub and gas flow dynamics models. Then, the reliability models of IES components are developed using the discretised-time Markov process to characterise the temporal state evolution in the operational horizon. A look-ahead contingency management scheme of the IES is then proposed to minimise the electricity and gas load curtailments. Taking account of all the possible system states, the operational reliabilities of the IES are evaluated using the time-sequential Monte Carlo simulation. Finally, the proposed method is validated by using the IEEE Reliability Test System and the practical Belgium gas transmission system [30].