The case examines whether sentencing juvenile non-homicide offenders to life imprisonment without parole violates the Eighth Amendment's Cruel and Unusual Punishments Clause. The Court considers the culpability of the offender, severity of the punishment, and whether the sentencing practice serves legitimate penological goals. Recent developments in psychology and brain science demonstrate fundamental differences between juvenile and adult minds, making the age of the offender and the nature of the crime relevant to the analysis. Life without parole for juvenile nonhomicide offenders lacks recognized goals of penal sanctions, including retribution, deterrence, incapacitation, and rehabilitation. The case for retribution is weaker with a minor as compared to an adult, and it becomes even weaker with respect to a juvenile who did not commit homicide. The Court has upheld severe but not grossly disproportionate sentences to the crime.

The Supreme Court held that the Eighth Amendment's prohibition on cruel and unusual punishment requires a proportionality review of a life sentence without parole for a juvenile offender who did not commit homicide. Justice Thomas dissented, arguing that the Court's decision is inconsistent with prior cases. However, Justice Stevens notes that the dissent's rigid interpretation of the Amendment fails to account for the evolving standards of decency and the need for proportionality review to prevent cruel and unusual punishment. The Court rejects Justice Thomas' static approach to the law and recognizes that standards of decency continue to evolve.

The Supreme Court found that a sentence of life without parole for a non-capital offense imposed on a juvenile violates the Eighth Amendment's prohibition on cruel and unusual punishment. The Court emphasized the importance of the legislature's role in setting sentences and the need for objective factors to guide review when applying the "narrow proportionality" analysis. Juvenile status can play a central role in the inquiry, given that juveniles are typically less blameworthy than adults. The Court established a new constitutional rule that a sentence of life without parole imposed on any juvenile for any non-homicide offense is unconstitutional. However, a case-by-case approach to proportionality review is necessary to distinguish the few incorrigible juvenile offenders from the many who have the capacity for change.

The Supreme Court has ruled that a life-without-parole sentence for a person under 18 who commits a non-homicide offense is always "grossly disproportionate" and violates the Eighth Amendment's Cruel and Unusual Punishments Clause. Justice Thomas dissented, arguing that this sentencing practice is allowed by Congress and many states and is not prohibited by the Constitution. The Court's decision to use categorical rules for non-capital sentences is inconsistent with the Framers' intent and expands its reach. However, the Court has established a categorical rule prohibiting life-without-parole sentences for all juvenile non-homicide offenders, which goes beyond what is justified by the Eighth Amendment or the Court's precedents. The Court's decision is not solely based on objective factors such as legislation and frequency of use, but also on the independent moral judgment of the Court.

Justice Alito dissents from the majority opinion and agrees with Justice Thomas on certain points. He clarifies that the Court's ruling only applies to juvenile offenders who did not commit homicide and does not affect the imposition of a sentence to a term of years without the possibility of parole. He also notes that the petitioner's sentence violates the narrow, as-applied proportionality principle is not properly before the Court because the petitioner did not include an as-applied claim in his petition for certiorari or in his merits briefs. Therefore, Justice Alito would not reach that issue.