Justice Breyer's remedial opinion in United States v. Booker not only rendered the Federal Sentencing Guidelines advisory but also called on appellate judges to ensure that sentences are not "unreasonable." Eighteen months after Booker, the appellate courts are still grappling with how to determine whether a sentence is reasonable or not. This month, four authors--Judge Nancy Gertner, Professors Doug Berman and Steve Chanenson, and Yale Law Journal Editor Eric Citron--offer their perspectives on the definition and practical meaning of appellate review of sentencing.