Fernandez sought compassionate release by arguing that intervening law showed that his conviction or sentence was legally invalid. The Supreme Court held that the compassionate-release statute is not an alternative vehicle for collaterally attacking a federal judgment. Claims that a conviction or sentence was imposed unlawfully must proceed through 28 U.S.C. § 2255 and its procedural limits. An asserted legal defect in the judgment therefore does not itself constitute an extraordinary and compelling reason under 18 U.S.C. § 3582(c)(1)(A). Because Fernandez’s request depended on a collateral challenge that belonged under § 2255, the judgment denying relief was affirmed.