This book addresses several aspects of the changing context of natural gas in the U.S. electric power sector. Increasingly plentiful and affordable natural gas has catalyzed major changes in U.S. power generation and has helped to boost U.S. economic recovery. Increased substitution of natural gas for coal in power generation has also cut U.S. GHG emissions. However, processes to produce natural gas, shale gas in particular, have also elevated environmental and safety concerns in certain regions of the country. The rapid rise of natural gas is also beginning to drive more thought on longer-term energy policy issues such as the appropriate level of generation diversity (given the history of volatile prices for natural gas), and trajectories of natural gas use that will still allow GHG mitigation sufficient to address the climate challenge. This book is intended to help inform these energy policy and investment discussions, and outlines the current dynamics of natural gas in the power sector and the selected challenges and opportunities in the use of natural gas to generate electricity.