The legal treatment of non-custodial parents has become a lightning rod in modern family law. The topic is obviously important. Every year in this country, about one million children see their parents divorce. In addition, roughly a third of all children are born to parents who are not married. Many of these parents, of course, will never live together, and those who do face a high risk of breaking up before the children are grown. Taken together, the trends suggest that fewer than half of all children born today—and perhaps as few as a quarter—will live with both their parents throughout childhood. Defining the custodial rights of divorced or separated parents is therefore likely to be a matter of direct concern for a majority of the nation’s children.