Peugh v. Davis is a United States Supreme Court case that addressed the issue of retroactive application of the United States Sentencing Guidelines. The case was brought by petitioner, Charles Peugh, who was convicted of bank fraud and sentenced to a prison term of 70 months. Peugh argued that the sentence he received was based on an outdated version of the Sentencing Guidelines, which had been amended after his conviction. He argued that the amended version of the Guidelines should have been applied to his case, and that the sentence he received was therefore unconstitutional. The Supreme Court agreed with Peugh, ruling that the Sentencing Guidelines must be applied retroactively. The Court held that the Guidelines must be applied to all cases, regardless of when the crime was committed, and that the Guidelines must be applied in a manner that is consistent with the Constitution. The Court also held that the Guidelines must be applied in a manner that is consistent with the principles of fairness and justice. The Court's ruling in Peugh v. Davis established a precedent that the Sentencing Guidelines must be applied retroactively, and that the Guidelines must be applied in a manner that is consistent with the Constitution and the principles of fairness and justice. This ruling has had a significant impact on the criminal justice system, as it has ensured that all defendants are treated fairly and justly, regardless of when their crime was committed.

Justice Field delivered the dissenting opinion in Peugh v. Davis, arguing that Congress had not intended to apply the Ex Post Facto Clause of the Constitution to criminal sentencing guidelines. He argued that it was clear from both legislative history and precedent that Congress did not intend for such a broad application of this clause, which is meant to protect individuals from retroactive laws being applied against them. Furthermore, Justice Field argued that applying this clause would be an unwarranted extension of its scope and could lead to unintended consequences in other areas where ex post facto laws are applicable. Finally, he noted that if Congress wanted these guidelines to be subject to constitutional scrutiny then they should have made their intentions clearer when passing legislation related thereto.