This thesis examines the question of whether the Iraqi High Criminal Tribunal, as a domestic court relying on international law, respected the principle of nullum crimen sine lege when prosecuting the Ba’ath Party legacy of internal displacements under the heading of forcible transfer as an international crime per se. If it did not do so, then which other international criminal frameworks, the thesis enquires, would have been adequate and valid? The research focuses on internal displacements in the cases of Al-Dujail, the Marshlands population and the Al-Anfal campaigns. These cases are significant in relation to the aforementioned principle, especially since there are some that have yet not been tried. The research conducted doctrinal legal research, and employed both primary and secondary resources: scholarly writings and publications, case law, Iraqi laws, instruments of international tribunals, international conventions and reports. The key findings establish that the reliance on international law to criminalize the Ba’ath Party legacy did not challenge the principle of nullum crimen sine lege. However, this was not the case with the criminalization of acts of internal displacement under the heading of forcible transfer as an international crime per se, whether under the category of crimes against humanity or that of war crimes in internal armed conflict. The research findings are that the Iraqi High Criminal Tribunal derived these categories from the Rome Statute and applied them retroactively. This Statute, however, entered into force only in July 2002, and forcible transfer was not recognised as a category of crime under international law during the periods when the Ba’ath Party abuses took place. Under international law at that period, forcible transfer was recognised as a war crime only in international armed conflicts; as a sub-heading of the crime of apartheid, and as a sub-heading of the crime of genocide through the transfer of children. These three exemplars were not applied to the Iraqi cases, and thus there is a considerable gap between the practice of the Iraqi High Criminal Tribunal and international law. The research therefore suggests alternative criminal frameworks: it demonstrates that the crime of persecution, the crime of other inhumane acts, and the crime of genocide through the sub-headings of both ‘causing serious bodily or mental harm’ and ‘inflicting conditions of life to bring a group about its physical destruction’ can serve to criminalize the Iraqi cases of internal displacement, particularly since these crimes were well established in customary international law and/or treaty law at the material time. The research concludes that violation of the principle of nullum crimen sine lege threatens the legitimacy of the Iraqi trials. It is therefore recommended that Iraqi legislators and judges should take the opportunity to amend Iraqi law and the Statute of the Iraqi High Criminal Tribunal to ensure that trials dealing with the Ba’ath legacy, or with future atrocities, are in line with the principle of nullum crimen sine lege and with international law. Finally, this work concludes with some suggestions that would help to ensure that similar trials, procedures, punishments and other criminal acts or frameworks in the future do not violate the principles of criminal law.