The G20 last met in Washington in November
The Group of 20 of the world's most powerful countries meets in London for a one-day summit on 2 April. The G20 aims to boost confidence that global leaders are united in tackling the world economic crisis. Among the key issues that will be discussed are:
REVIVING THE WORLD ECONOMY
The key aim of the summit is to agree global action to do "all that it takes" to boost the world economy.
Governments are being urged to take "coordinated action" to stimulate their economies by boosting spending.
The International Monetary Fund (IMF) has suggested that a fiscal stimulus of at least 2% of GDP, amounting to some $2 trillion (£1.36trn), will be needed to counteract the effects of the slowdown.
But its latest figures show that although some big countries, including China and the US, are meeting this target, others, including France and Japan, are not.
Some European countries have suggested that because their economies get an automatic boost from a government cash injection when a slowdown occurs - since they have to pay out more money in benefits such as unemployment - they are already meeting the targets.
But even with these so-called "automatic stabilisers", IMF figures suggest that by 2010, their will only be a weak global stimulus.
However, there will be an argument that cuts in interest rates, or other actions by central banks to boost the money supply, are also a key part of any economic stimulus package.
One of the key problems that is depressing world growth is the continuing trouble in the banking system.
Banks still hold many "toxic" assets, many based on risky mortgages, that they are not able to value or sell.
So the G20 is likely to push for quick action by governments to resolve the situation.
But action has been slower than hoped for.
The US has unveiled a plan for a private-public investment fund to assist the purchase up to $1trn in toxic assets held by US banks, while only some banks in the UK have joined the Bank of England's asset protection scheme. Meanwhile, there has been a piecemeal approach to bank bail-outs in Europe.
The slowdown in bank lending has particularly hit developing countries hard, with private sector flows of money falling off sharply.
The underlying cause of the slowdown, many people believe, was the lax regulation of banks and other financial institutions.
So the G20 summit will try to strengthen the international regulation of banks to prevent future crises.
This is likely to include requirements to force banks to hold more money (capital) against the possibility of future losses.
And all financial institutions that could pose a risk to the system will be regulated.
There will be plans for global oversight of the risks that the financial system is running as a whole, so-called "macro-prudential regulation".
Some governments also want a common set of rules governing compensation, to prevent banks paying big bonuses that encourage risky behaviour.
And there will be a drive to ensure that all jurisdictions, including the so-called off-shore financial centres or tax havens, are included in any new global deal.
BOOSTING THE IMF
A major aim of the G20 summit is to beef up the finances of the IMF, which is running short of cash.
The IMF has already bailed out seven countries in the past six months, to the tune of $46bn (£32.7bn), and more are in the pipeline.
That means it has about $150bn left to lend this year.
UK Prime Minister Gordon Brown wants to make at least $500bn available to the IMF. Japan has already stepped up to the plate, with a $100bn loan, and the European Union has offered an additional $100bn.
However, developing countries, including China, are likely to be reluctant to stump up cash unless the IMF members agree to reforms that give emerging nations more say in how the body is run.
They would also like a permanent increase in the IMF's resources, perhaps by letting it create its own currency.
One option that may be discussed is a new policy-setting council that would shift power away the US and EU and towards rising economic powers.
The IMF is also likely to be given more powers to monitor risks within the banking system.
The world's poorest countries are likely to be hard-hit by the downturn.
The World Bank estimates that an extra 53 million people will fall into poverty because of the global recession.
But there is concern that many countries are now likely to cut their development aid.
Gordon Brown would like world leaders to pledge to maintain that aid, and if possible to increase it in line with targets agreed at the Gleneagles summit in 2005.
However, development groups such as the Overseas Development Institute say that at least $50bn more is needed for sub-Saharan Africa to escape the worst effects of the crisis.
There is a big worry that in an economic slowdown countries will turn to protectionism, trying to help their own workers by blocking free trade.
The G20 summit is likely to re-affirm the commitment of world leaders to open trade.
And it will probably call for the resumption of the world trade talks, known as the Doha round, that are aimed at freeing up agricultural trade to benefit poor countries.
Despite this rhetoric, the World Trade Organisation has reported that many countries have already have resorted to protectionist measures in the downturn.
Devising a mechanism for monitoring free trade will be a more difficult challenge for the summit.