wishingtoenterintotheInternationalCriminalCourt(ICC)regime,andtotakefull advantage of the principle of complementarity, to adjust their national law to the requirements of the Rome Statute. This can be a one, two or several step process, and the legal and constitutional obstacles and challenges vary widely from state to state. Particularly encouraging are early signs that some states have been prepared togobeyondthesubstantivelawaslaiddownintheStatuteto,forinstance,provide 4 broader definitions of crimes. Ratification and implementation of the ICC Statute presents a unique opportunity for states to contribute to the progressive develop- 5 mentofcustomaryinternationalcriminallaw. Giventhefactthatratificationofthe Statute has been proceeding apace, and that it is expected to come into force in 2002, much more activity in this sphere can be expected in the next few years. We may then see the emergence of layers or parallel systems of criminal enforcement of humanitarian law: by ad hoc international courts, the ICC and by states.