UN Secretary General Kofi Annan told the General Assembly it must consider radical reforms in the face of increased threats to global security. BBC diplomatic correspondent Barnaby Mason answers questions about the significance of Mr Annan's speech.
What is Kofi Annan suggesting?
The key UN institution for Mr Annan is the Security Council, the body charged with maintaining international peace and security. He wants it enlarged so that it reflects better the realities of the modern world.
Mr Annan says the Council has to demonstrate that it can deal effectively with difficult issues, including that of individual states using force pre-emptively to head off looming threats.
He suggests the Security Council should discuss in what circumstances it might authorise such action - for example against terrorist groups armed with weapons of mass destruction.
Mr Annan also favours strengthening the General Assembly, where the big powers cannot exercise a veto, though he did not say how.
Why has he spoken out now?
The proposals are clearly a response to the shock delivered to the UN system by US President George W Bush when he invaded Iraq without authorisation from the Security Council.
While condemning that action, Mr Annan is also considering how to bring future military responses by the United States back under the UN umbrella.
What reception are his proposals likely to receive among the key players?
Most countries say they favour enlarging the Security Council; the problem is how to do it.
Britain recently gave a new push to the idea. France has now backed the idea of far-reaching UN reform (provided it does not involve the loss of its own veto power) and can be expected to favour the idea of new rules governing the use of pre-emptive force.
The same probably goes for Russia. On the other hand, the Bush administration consistently opposes any international restraints on its right to defend the interests of the United States.
How feasible is reform of the UN?
As Mr Annan pointed out, expanding the Security Council has been on the agenda for more than a decade. He told world leaders that in the eyes of their peoples the difficulty of reaching agreement did not excuse their failure to do so.
The main problems are deciding which Asian, African and Latin American countries should be chosen as new permanent members; and the level of European representation - given that Britain and France are already there.
As for strengthening the General Assembly, the big powers would oppose giving it powers that would challenge those of the Security Council. So the chances do not look particularly good.
What are the likely next steps?
Mr Annan says he will set up a high level panel to examine current challenges to peace and how to address them collectively. It will also review the working of UN bodies and how they might be reformed.
He has asked the panel to report in time for him to make recommendations to next year's General Assembly. It is clear, as always, that nothing will be done in a hurry.