States have undertaken an international obligation to investigate and prosecute individuals for serious human rights violations and grave breaches of international humanitarian law. However, compliance with that obligation is poor and prosecutions at the national level remain few. The mechanism for enforcement of that obligation is also limited. This article explores the way in which the International Court of Justice (ICJ) can play, and has played, a role in this respect. The jurisprudence of the Court is analysed with regard to three matters: (i) the obligation of states to investigate and prosecute serious crimes at the national level; (ii) national criminal jurisdiction with regard to prosecution of serious crimes, as well as immunities from that jurisdiction; and (iii) the obligation of states to cooperate in criminal matters with other jurisdictions. The Court has adjudicated on some key issues relating to national prosecutions. Some of its findings have, without doubt, enhanced the enforcement of prosecution at the national level, while others have undermined it. Recent cases before the ICJ show an increased willingness by states to use the Court as an avenue for enforcement and, at the same time, the Court has proved more willing to utilise its powers.