The United States (US) Constitution’s Eighth Amendment includes a restriction on cruel and unusual punishments. Over the past fifty years the punishments clause has been developed by the US Supreme Court through its ‘evolving standards of decency’ (ESD) jurisprudence, restricting the range and application of lawful capital and non-capital penalties. Although the punishments clause has been evolved in the capital sphere such that the American death penalty is reaching a vanishing point, the Court has neglected to apply similar scrutiny in the non-capital setting, especially with respect to conditions of imprisonment. By undertaking an examination of the Eighth Amendment, a theoretical framework is developed in order to understand how the ESD principle has been applied, and to examine how a future constitutional challenge to disproportionate confinement conditions might materialise. This thesis contends that modern solitary confinement represents a recession of constitutional protection. It is argued that principles of morality underlying the Eighth Amendment create a bar to this severely disproportionate, under-reviewed, and often under-reported punishment. In reaching such a conclusion, Dworkin’s theory of interpretivism is applied to solitary confinement in a novel way. An interpretivist understands morality to have been an undercurrent in the drafting, adoption, application and, therefore, future interpretation of the Constitution. Moral principles trump majoritarian policies, and such an approach compels a curtailment of extreme solitary confinement under the Eighth Amendment’s ESD principle. Sources of morality relied on to reach such a conclusion are derived from the community and include traditional consensus, which is state counting, in addition to other elements selected for analysis due to their regular citation in Eighth Amendment decisions: public opinion, penological principles, transnational perspectives, and professional consensus. As a result, an original contribution is also made to the medico-legal literature, which has traditionally fixated on the psychiatric implications of confinement. Wider implications will extend to other areas of academic commentary, including professional consensus literature, and transnational law.