What is the Johannesburg Earth Summit?
The World Summit on Sustainable Development - to give its full name - is the follow-up to the Rio Earth Summit in 1992. That was a follow-up to the Stockholm Conference on the Human Environment in 1972. The summit is a new attempt at planet management.
When does it get going?
The meeting officially runs from Monday to Sept 4. World leaders, including Tony Blair, are expected to attend for the last two days. Margaret Beckett, Environment Secretary, will be there for the duration, with other ministers, including John Prescott, Michael Meacher and Claire Short attending for a few days at a time. Around 60,000 delegates will attend.
What came out of Rio?
The Convention on Biological Diversity, which aims to protect endangered wildlife. As US president, George W Bush's father refused to sign this treaty and America is still not a party to it.
The Convention on Climate Change. This committed signatories to freezing their emissions of greenhouse gases at 1990 levels by 2000. Hardly anyone did this. The convention spawned the Kyoto Protocol, requiring legally binding emission reductions, yet to come into force.
President Bush has said America will have nothing to do with it until at least 2012.
Agenda 21, setting out environmentally desirable actions for the waste, chemicals, water, forestry and agriculture industries, non-governmental organisations and local government.
The Rio Declaration, the soul of the summit. A 27-point declaration that enshrines key concepts of international environmental policy, including the idea that industrial nations should take action first to solve problems they have caused like climate change.
The US Senate remains implacably opposed to this principle.
What may come out of Johannesburg?
A new political declaration. An implementation plan, drawing together commitments from the Rio summit as well as more recent gatherings.
Key environmental issues still not agreed include measures on sustainable consumption, removing environmentally harmful subsidies, increasing the use of renewable energy sources and halving the number of people without access to safe drinking water and sanitation by 2015.
Partnerships between rich and poor countries, and between companies and countries or other groups. Monsanto and other biotech firms are known to want to enter some of these.
How effective are these international conferences?
According to a report by the University of East Anglia mega-conferences on the environment may not achieve much politically, but can lead to changes in people's lives and thinking long after the event.
Dr Gill Seyfang and Dr Andrew Jordan examined environmental mega-conferences of the past 30 years and pointed out that there were no effective sanctions against counties that reneged on their promises, as America did with the Rio Convention and the Kyoto Protocol.
But Dr Jordan, an expert on environmental regulation, said: "Mega environmental conferences do promote a long and slow change in attitudes, encouraging governments to adopt statements of principles or 'soft laws' which are then translated into more binding regulations and laws over time."