International criminal tribunals had to make a choice between the principles of opportunism and legalism or decide to use a mixture of these both. They had to decide whether a prosecutor should become “the minister of justice” (as in the principle of legalism) or “the first judge” (evaluating in the frames of principle of opportunism the reasonable basis for prosecuting). This article addresses prosecutorial discretion before the ICC with respect to selecting defendants. Firstly, it analyzes the main differences between opportunism and legalism of prosecution. It also presents models of accusation functioning before the historical and existing international criminal tribunals – which usually opted for opportunism of prosecution. Before the ICC the conditions on which the Prosecutor may initiate an investigation are set in Art. 53(1) of the Statute: “The Prosecutor shall, having evaluated the information made available to him or her, initiate an investigation unless he or she determines that there is no reasonable basis to proceed under this Statute.” It is interesting to observe that this phrase may be interpreted in many various ways, depending on the model of accusation the author belongs to: those coming from the Anglo-Saxon tradition have tendency to search for elements of opportunism; those from civil law states assume that the model of accusation operates according to the principle of legalism. There is also a number of mixed options presented, according to which the ICC operates according to a mixture of these two principles. Finally, the article presents different rules adopted by the ICC Prosecutor (or proposed), which govern the choice of the defendants.

The main topic of this article is retroactive application of procedural criminal law. In this text the question will be posed – and answered – whether the application of a new procedural provision that entered into force in the course of an ongoing proceeding should in that proceeding be considered as retroactive and in what scope or/and under what conditions can such retroactivity be allowed for. As will be shown the solutions in national jurisdictions differ according to the common law – continental law states divide. This problem will be discussed in the light of a decision in the ICC Ruto and Sang case. In this case the ICC Appeals Chamber had to answer several questions pertaining to the temporal application of new procedural provisions. Firstly, the Chamber had to decide whether a general ban on the retroactive application of substantive law should also apply to procedural criminal law. Secondly, the ICC Appeals Chamber had to analyze the criteria according to which it would evaluate whether the change of rules of criminal procedure in the course of an ongoing trial was to be considered as having a retroactive effect, and whether the change in the rules of admission of evidence could be considered detrimental to the accused. Thirdly, it will be shown that the ICC Appeals Chamber has chosen the common law concept of “due process rights” rather than the idea of “intertemporal rules” known from the continental doctrine, and why it chose to do so.