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The Ex Post Facto Clause bars any increase in punishment after the commission of a crime. But deciding what constitutes an increase in punishment can be tricky. At the front end of a criminal case, where new or amended criminal laws might lengthen prisoners’ sentences if applied retroactively, courts have routinely struck down such changes under the Ex Post Facto Clause. At the back end, however, where new or amended parole laws or policies might lengthen prisoners’ sentences in exactly the same way if applied retroactively, courts have used a different standard and upheld the changes under the Ex Post Facto Clause. Because the harm is identical and lies at the core of what the Ex Post Facto Clause is supposed to protect against, we think the asymmetry is mistaken. Parole is an integral part of punishment: it determines how much time people will serve on their sentences. Until the twenty-first century, black-letter law forbade even modest parole changes that were adverse to prisoners. If a change in the parole regime might lead to longer sentences, then courts insisted that the change be applied prospectively only. Over the last two decades, relying on language in two US Supreme Court parole cases decided in 1995 and 2000, the lower courts have shifted parole ex post facto doctrine by 180 degrees. Prisoners can no longer prevail, even when the change in the state parole regime is almost certain to lead to significantly longer sentences. In the context of parole, the courts have repudiated past doctrine and strayed far from the purposes of the Ex Post Facto Clause. In this article, we review the history, show how the current case law is misguided and illogical, and put forward a new framework that would restore the Ex Post Facto Clause to its rightful place.