The common law principle of double jeopardy prohibits a second trial where a defendant has been formerly acquitted (autrefois acquit) or convicted (autrefois convict) of the same criminal offence. In civil law systems it is known as ne bis in idem. Reform in the UK has been the model for reform in other common law jurisdictions including Ireland, Australia, and New Zealand. Henceforth, an acquittal might be quashed based on new and compelling evidence of the defendant’s guilt. Implications for the criminal justice process include investigative and prosecutorial (in)efficiency, adverse pre-(re)trial publicity, (in)sufficiency of due process rights, exclusionary rules of evidence, sentencing, the standing of victims, and the overall integrity of the criminal justice process. The purpose of this project is to critically evaluate statutory modification of the common law principle against double jeopardy and consider the international dimension and Ireland’s obligations under European and international law.