The demographic transformation sweeping the emerging world has profound implications for U.S. development policy. The challenge is no longer helping countries overcome the obstacles to development posed by high birthrates and rapid population growth, but leveraging the opportunities created by falling birthrates and slowing population growth. This report discusses how developing countries can best leverage their demographic dividends in order to boost income and wealth while they are still young and growing, as well as how they can prepare for the inevitable aging of their populations that looms just over the horizon. It also explores what the emerging new demographic realities imply for the optimal shape of U.S. development policy in decades to come.