BBC News examines issues surrounding the High Court's ruling that government plans for new runways at Heathrow and Stansted airport are lawful.
Stansted Airport will gain a second runway under the government plan
What exactly has the High Court decided?
The High Court has decided that government plans for airport expansion in the South East are lawful.
However, the court stipulated that residents and local authorities near Stansted airport in Essex should have a say in where a new runway will be built and how much land it will take up.
The court said that while a second runway at Stansted was "a fair outcome of the consultation process", the statement that the runway would
be "the wide-spaced runway option presented in the consultation document" was unfair.
There must also be further consultations over expansion at Luton Airport in Bedfordshire.
The court rejected claims by residents that plans for a third runway at Heathrow were unlawful.
What is the reaction to the decision?
The Stop Stansted Expansion (SSE) group has hailed the verdict as a success, adding that it represents a "major setback" for the UK airport firm BAA, which intends to build a new runway at Stansted by 2012.
The SSE, however, has said it will continue to campaign against the second runway.
Hertfordshire County Council leader Robert Ellis
said the Luton ruling was "a tremendous result for local people".
The Transport Secretary Alastair Darling said he was pleased with the ruling and that the airport expansion policy was "on
BAA chief executive Mike Clasper said the judgement "ensures that the government's White Paper will be the bedrock of UK aviation policy for the next 30 years".
Why does the government want to expand the airports?
A massive increase in air traffic is predicted by 2030, especially in Britain's heavily populated South East.
South East airports are used by 120 million of the 200 million UK passengers flying each year.
As one can imagine, Heathrow is particularly important.
London is the biggest tourist destination in the country and other popular sights, such as Oxford and Cambridge, are nearby.
Business travellers use London airports for easy access to the City, and Heathrow is an important airport for passengers transferring from long-haul international flights, to short-hop European ones.
For all these reasons, London airports are vital for the airlines, which want to maximise the facilities they offer.
What will be the environmental impact at Heathrow and Stansted?
It will differ greatly, which is why the government has chosen Stansted over Heathrow for the first new runway in the South East.
Air pollution is the biggest issue. It is estimated the rural location of Stansted would mean just 20 people are affected by higher levels of pollutants like nitrogen dioxide, while at Heathrow the number is up to 30,000.
As for noise pollution, the Heathrow expansion will only be allowed if it does not increase the number of people affected by noise.
At Stansted up to 14,000 people will suffer from higher levels of noise.
How will Heathrow stay within the pollution limits?
Opinion varies. Many environmentalists and pollution experts believe Heathrow will never meet European pollution limits due to be introduced in 2010.
Not only will flight numbers grow steadily but so will traffic levels on surrounding roads, including Britain's busiest motorway, the M25, and the M4.
To meet the targets, BAA, which owns Heathrow, will have to introduce low-emission service vehicles, and persuade airlines to use new, cleaner aircraft engines.
Heathrow's new runway will not be built unless the company succeeds.