This chapter discusses changes in American law in the twentieth century. Change in the twentieth century was, in some ways less dramatic than those in the previous century; in other ways, more so. The United States now dominated large portions of the world, even when it did not actually own these far-off places. At home, the population grew enormously; according to the 2000 census, the population was just over 280 million, and by 2016, grew to something over 325 million. The main engines of revolution were social and technological. The technological revolution was, perhaps, a chief cause of the social revolution. This was the century of the automobile and the airplane, radio, the movies, and television, the computer and the internet, antibiotics and the birth control pill. Each of these great advances in science and technology eventually had a deep impact on society and on law.