Over the last half century, the body of international law that enshrined the sovereign independence of states has given way to new legal understandings that obligate governments to protect human rights. But does this emerging framework really constrain states? Simmons says yes, and proceeds to offer an extraordinarily rich and sophisticated argument about why, when, and how. Skeptics argue that governments sign agreements that obligate them to follow principles to which they already adhere or that they can easily ignore. Simmons agrees that governments pick and choose treaties, but over the long term, she sees human rights agreements changing the politics inside countries. They influence legislative agendas, alter political coalitions, and define the terms of acceptable state action. Simmons marshals impressive empirical data to test her argument and to distinguish it from alternative stories, examining the record in areas such as civil rights, the equality of women, and the humane treatment of prisoners. Her pathbreaking book explains much about the relationship between international law and national politics, but it also provides an inspiring glimpse of human progress in action.