The United States, the third most populous country globally, accounts for about 4.5% of the world's population. The U.S. population, currently estimated at 308.7 million persons, has more than doubled since its 1950 level of 152.3 million. More than just being double in size, the population has become qualitatively different from what it was in 1950. As noted by the Population Reference Bureau, "The U.S. is getting bigger, older, and more diverse." This book provides an overview of the inexorable demographic momentum taking place, which will have important implications for the economic and social forces shaping future societal well-being.