The International Criminal Court has been ratified by 99 countries - but crucially not by the United States.
The court has universal jurisdiction to prosecute genocide, crimes against humanity and war crimes.
BBC News Online examines the main issues behind the creation of the court.
What is the court designed to do?
To prosecute and bring to justice those responsible for the worst crimes - genocide, crimes against humanity and war crimes - committed anywhere in the world.
It is a court of last resort, intervening only when national authorities cannot or will not prosecute.
Aren't there already several international courts?
Yes, but they either do different jobs or have a limited remit.
The International Court of Justice (sometimes called the World Court) rules on disputes between governments. It cannot prosecute individuals.
The international criminal tribunals for the former Yugoslavia and Rwanda do try individuals for crimes against humanity, but only those committed in those territories over a limited time.
The tribunals will eventually be wound up. The new International Criminal Court, however, is a permanent body.
Are there any time limits on what it covers?
First of all, the court has no retrospective jurisdiction - it can deal only with crimes committed after 1 July 2002 when the 1998 Rome Statute came into force.
Then, the court has automatic jurisdiction only for crimes committed on the territory of a state which has ratified the treaty; or by a citizen of such a state, or when the United Nations Security Council refers a case to it.
How does the system work?
The prosecutor begins an investigation if a case is referred either by the Security Council or by a ratifying state. He or she can also take independent action, but prosecutions have to be approved by a panel of judges.
Both the prosecutor and the judges are elected by the states taking part in the court. Luis Moreno-Ocampo of Argentina is the first chief prosecutor of the court.
Each state has a right to nominate one candidate for election as a judge.
Eighteen judges of the court elected the presidency on 11 March 2003.
It is composed of Judge Philippe Kirsch (Canada) as President, Judge Akua Kuenyehia (Ghana) as First Vice-President, and Judge Elizabeth Odio Benito (Costa Rica) as Second Vice-President of the court.
Who has agreed to co-operate with the court?
Ninety-nine states have ratified the Rome Treaty so far - and have therefore bound themselves to co-operate - of the 139 that have signed and may ratify it in the future.
Only one Arab state has joined so far - Jordan.
Why isn't the United States involved?
During the negotiations on the treaty, the Americans argued that their soldiers might be the subject of politically motivated or frivolous prosecutions.
Various safeguards were introduced partly to meet this objection.
Bill Clinton did eventually sign the treaty in one of his last acts as president; however, the Bush administration has been adamantly opposed to the court and to any dilution of American sovereignty in criminal justice.
The US threatened to pull its troops out of the UN force in Bosnia unless they were given immunity from prosecution by the ICC. In a much-criticised decision, the Security Council voted on 12 July 2002 on a compromise that gave American troops a 12-month exemption from prosecution - renewed annually.
But the Security Council - prompted by Secretary General Kofi Annan - refused to renew the exemption in June 2004 - two months after pictures of US troops abusing Iraqi prisoners shocked the world.
The court's operation is seen as weakened without US involvement.
However, Washington's co-operation with the court in particular cases has not been ruled out, as in the case of Sudan's western Darfur region.
The referral of the case to the ICC, the first by the Security Council, was made possible after the US backed away from using its veto. Washington was given guarantees that its own citizens in Sudan would be exempt from prosecution.
Are there other dissenters?
Yes, a number of important countries seem determined not to submit to the jurisdiction of the International Criminal Court.
Some have not even signed the treaty, such as China, India, Pakistan, Indonesia, and Turkey.
Others have signed but remain dubious and have not ratified, for example, Egypt, Iran, Israel and Russia.
It is unlikely that alleged crimes against humanity in those states will be prosecuted.
How does it fit in with each nation's judicial system?
States that join the treaty may want to make sure that they themselves are able to prosecute all the crimes that it covers - otherwise the court may intervene.
Some governments have already introduced legislation to make changes to their own judicial systems.
Who is paying?
The states which take part. This will be according to the same rules that govern their contributions to the UN - roughly based on their national wealth.
The absence of the US and Japan in particular will make the funding of the court more expensive for others.
Germany, France and Britain will be the largest contributors, at least at first.