Accurate assessment of witness testimonies underpins judicial fact-finding at international criminal courts and tribunals (ICCTs). However, the lack of formal assessment criteria and uncoordinated methods, coupled with advances in the scientific understanding of the psychology of witnessing, calls for a re-examination of the judicial practice. This study critically evaluates the state of the art of witness assessments at the International Criminal Tribunal for Rwanda (ICTR), International Criminal Tribunal for the former Yugoslavia (ICTY), and the International Criminal Court (ICC),based on all the trial judgments issued in 1996-2019. The analysis results in a consolidation of this ad hoc, constantly evolving jurisprudence, into a framework that has been in development since the 1990s. The authors reflect upon the scientific validity of the criteria used throughout the analysis, based on up-to-date findings from psychology and criminology, and identify the areas that would most benefit from standardized procedures.