The increased use of renewable electric power generation, particularly wind, will require electric power systems to provide back-up power to firm the generation from these intermittent sources of electricity. Natural gas-fired generation is a logical, low-cost choice for providing this back-up firming capability. This study evaluates the implications of the increased use of natural gas-fired generation for firming renewable resources for natural gas transportation infrastructure planning and pricing. Firming capacity in this study is the amount of non-wind generating capacity needed to meet shortfalls in actual wind output with respect to forecast wind output, that is, to compensate for the forecast uncertainty.