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 Peugh argued that the sentence he received was based on an outdated version of the Sentencing Guidelines, which had been amended after his conviction.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, which would have resulted in a shorter sentence. 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. The Court reasoned that the Guidelines are intended to provide uniformity and fairness in sentencing, and that retroactive application of the Guidelines would ensure that all defendants are treated equally. The Court also noted that the Guidelines are not intended to be punitive, and that retroactive application of the Guidelines would not result in an increase in the severity of the sentence. The Court concluded that retroactive application of the Guidelines was necessary to ensure that all defendants are treated fairly and uniformly.

Justice Stephen Field delivered the dissenting opinion in Peugh v. Davis, arguing that Congress had violated the Ex Post Facto Clause of the United States Constitution when it retroactively applied a harsher sentencing guideline to Peugh's crime. The majority held that because there was no "significant risk" of increased punishment for crimes committed before the new guidelines were enacted, there was no violation of ex post facto principles and thus affirmed Peugh's sentence. Justice Field disagreed with this conclusion, noting that while an increase in punishment may not be significant enough to constitute a constitutional violation under certain circumstances, such as when a defendant is already serving time for another offense or has been convicted multiple times for similar offenses, this did not apply in Peugh’s case since he had only been convicted once and his sentence would have been significantly lower if he had committed his crime after enactment of the new guidelines. He argued further that even though Congress could enact laws which provided more lenient sentences than those previously established by law without violating ex post facto principles, they could not do so at any cost; rather they must take into account whether their actions will result in unfairness or injustice towards individuals who are subject to them retrospectively.