International criminal law has made impressive strides over the past twenty years. The 1990s and 2000s saw the establishment of ad hoc international criminal tribunals for the former Yugoslavia and for Rwanda (ICTY and ICTR), the coming into being of the permanent International Criminal Court (ICC), and the birth of several internationalized ‘hybrid’ jurisdictions, notably the Special Court for Sierra Leone (SCSL) and the Extraordinary Chambers in the Courts of Cambodia (ECCC). However, the dynamic development of international criminal law into a new branch of public international law has also led to some problems and confusion. The rules and principles developed by the newly founded international criminal tribunals have sometimes seemed at odds with accepted views on public international law more generally – raising fears about the ‘fragmentation’ of the law. Perhaps the best-known example of this is the controversy over the ‘overall-control’ test developed by the ICTY Appeals Chamber in the Tadić case to determine under which circumstances armed forces may be considered to be acting on behalf of a third state, rendering an internal armed conflict international. The Tadić test differed from the ‘effective-control’ test developed by the International Court of Justice (ICJ) in the Nicaragua case and confirmed in the Bosnia Genocide case, where the ICJ specifically rejected the ICTY approach. Also more generally, the discussion on ‘fragmentation’ and international criminal law continues; recently, Elies van Sliedregt set out in these pages her vision of legal pluralism in international criminal law.