On September 25, 2005, John G. Roberts took the oath of office as the seventeenth Chief Justice of the United States. He replaced William H. Rehnquist, for whom he had once served as a law clerk. Roberts's confirmation marked the end of one era in Supreme Court history and the beginning of another. When I wrote the first edition of this book in 2004, Rehnquist had been the Chief Justice since 1986. More significant, in 2004 the composition of the Supreme Court had remained unchanged for ten years, the second-longest period of unaltered membership in the Court's entire history. Because the Supreme Court sits to resolve hard cases that divide the lower courts, change in constitutional law is a constant, even when the Justices stay the same. But in 2004, stable patterns had emerged in many, if not most, areas. It was possible to write with reasonable confidence about the doctrinal equilibria into which the Justices had settled.

The confirmation of John Roberts to replace William Rehnquist, swiftly followed by Samuel Alito's replacement of Sandra Day O’Connor, inevitably brought change. The Chief Justice is almost invariably the most influential of the Justices in the Court's internal workings. Among other things, the Chief Justice gets to decide who writes the majority opinion in any case in which he votes with the majority. The substitution of Justice Alito for Justice O’Connor was especially consequential, too. Through most of the years of the Rehnquist Court, Justice O’Connor was the “swing,” or median, Justice – the Justice whose views were most likely to tip the balance between the Court's more conservative Justices and its liberals. When O’Connor retired, to be replaced in 2006 by the more conservative Alito, the Court's center of gravity moved further to the right. There have been two further membership switches since then, with Sonia Sotomayor replacing David Souter and Elena Kagan succeeding John Paul Stevens.