The purpose of Twentieth-Century Science, a new seven-volume book set, is to provide students, teachers, and the general public with an accessible and highly readable source for understanding how science developed, decade by decade, during the century and hints about where it will go during the early decades of the 21st century. Just as an educated and well-informed person should have exposure to great literature, art, and music and an appreciation for history, business, and economics, so too should that person appreciate how science works and how it has become so much a part of our daily lives.