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.