Prior to the middle of the twentieth century, criminal procedure was essentially a branch of national law. It had developed into two systems regarded as fundamentally different: the ‘sport-match model’ of the common law which was party-driven and where the judge's role was akin to that of an umpire; and the ‘drill model’ of the civil law whereby the judge was responsible for finding the true facts. As far as evidence was concerned, the common law was characterised by detailed regulations and exclusionary rules, whereas the continental system adhered more strongly to the principle of free proof. These systems appeared to be irreconcilably opposed to each other, which is one of the reasons why it proved extremely difficult to pass treaties on mutual assistance in criminal matters between the United States and European states – Switzerland managed to conclude such a treaty in 1975, while it took until 2003 for Germany to achieve the same result.

Two developments in particular led to this branch of the law assuming importance at an international level: the emergence of international criminal proceedings, starting with the Nuremberg and Tokyo trials and culminating in the establishment of a permanent International Criminal Court on the one hand, and the development of a regional and international case law on the basis of human rights’ instruments on the other. Regarding international criminal tribunals, it was necessary to find solutions which would be truly international and could find acceptance both in countries adhering to the common law system and those following the European continental tradition. In the area of human rights, it was initially the European Commission and Court of Human Rights which were faced with the challenge of applying fair trial standards in a way that would make sense in the United Kingdom, Ireland and Malta, as well as in France, Germany and Italy.