This introductory chapter takes a brief look at family law in the United States as it changed over twentieth century and the start of the twenty-first. “Family law” refers to a particular branch of the law—mostly about marriage, divorce, child custody, family property, adoption, and some related matters. However, this chapter also briefly considers other parts of the law that touch on the family in an important way, such as inheritance or the intersection between criminal law and family affairs. The chapter then considers the changes to family law in this expanded sense. In part, the changes were continuations of trends that started in the nineteenth century; but in part they were completely new. Perhaps the single most important trend was the decline of the traditional family, the family as it was understood in the nineteenth century, the family of the Bible and conventional morality.