Heathrow Airport's £14bn plans for a third runway have been defeated by the Court of Appeal.
Or more importantly, the government's actions have been defeated.
Top judges ruled Transport Secretary Chris Grayling acted "unlawfully" when he drew up a National Policy Statement on airport expansion - and it must now be rewritten.
It throws plans to open a new runway by 2026 into chaos and sets up a Supreme Court fight.
But importantly it doesn't kill off Heathrow completely. Instead it throws the ball into Boris Johnson's court.
And that is a very tricky situation for our Prime Minister - who once vowed to lie down in front of the bulldozers to stop Heathrow's third runway.
Here's everything you need to know about today's ruling, and what happens next.
What was the case about?
A string of campaign groups challenged the government's National Policy Statement (NPS) - the government document that basically gave the green light to Heathrow, and which MPs backed 415-119 in 2018.
They brought four separate 'judicial reviews', claiming the proper legal processes weren't followed when the NPS was drawn up and voted on by MPs in 2018.
The campaigners previously lost a High Court fight in May 2019.
But today their cases were to be streamlined together into one mega-ruling at the Court of Appeal.
Together the various campaigns were challenging how the NPS followed (or didn't follow) the rules on habitats, access, noise, costs and climate change.
Issues on air pollution were removed from the case, to be dealt with by a planning inspector at a later date.
Which bit of the case did campaigners win?
Three top judges said the National Policy Statement was drawn up unlawfully.
This is - specifically - because bungling Transport Secretary Chris Grayling didn't take into account the Paris Agreement on climate change when he drew up the NPS.
The UN's Paris Agreement, which came into force in November 2016, commits signatories to tackling climate change by taking measures to limit global warming to well below 2C.
Other matters brought by the campaigners - including on habitats - did not succeed in court.
A challenge by Heathrow Hub Ltd, which wanted to extend the northern runway rather than build a new one, also failed and was denied permission to appeal.
Who brought the case?
The four cases were brought by Friends of the Earth; Plan B; Heathrow Hub Ltd; and Greenpeace together with the Mayor of London and five local councils.
What happens now - and will it stop Heathrow Airport?
Today's ruling does not - in itself - stop Heathrow Airport building a third runway.
Instead, it says the government document that allowed the runway was unlawful and must be rewritten.
Lord Justice Lindblom stressed: "We have not decided and could not decide that there will be no third runway at Heathrow. We have not found that a National Policy Statement supporting this project is necessarily incompatible with the UK's commitment to reducing carbon emissions... or with any policy."
That means the government could still push through Heathrow Airport expansion under a revised NPS.
But that will be a political decision for Boris Johnson - who of course vowed to block the third runway.
The government did not ask for permission to appeal to the Supreme Court - which would have the final word. But Heathrow Airport will launch that appeal. An airport spokesman said: "We are confident that we will be successful."
Meanwhile this protracted row could delay the third runway - which was supposed to open in 2026.
What does this mean for Boris Johnson?
This ruling puts Boris Johnson in a very tricky position.
The MP who once vowed to lay down in front of the bulldozers fled to Afghanistan to avoid voting on the NPS in 2018 - and has avoided being pinned down on the issue.
Now, however, his big excuse for holding back - that legal cases were ongoing - has fallen down.
Does he side with the campaigners and seize the moment to block Heathrow?
Or does he push through a new National Policy Statement? And if so, how will he vote on it?
It appears Mr Johnson is unlikely to sanction a fight against today's decision at the Supreme Court. The PM's spokesman said yesterday that Heathrow still had to prove it can meet obligations on noise and air quality.
When was Heathrow built?
Heathrow Airport is an old aerodrome, expanded time and time again, on what was once the hamlet of Heath Row in West London.
It opened for commercial operations as London Airport in 1946.
It has two runways but runs almost entirely at capacity, especially after Terminal 5 opened in recent years.
A plan to expand its runway capacity has been rolling on in some form since 2001.
What was the plan?
Heathrow Airport planned to build a third runway to the north west of its existing ones in a £14bn project opening in 2026. There would also be a Terminal 6.
See the plan in maps here.
It's designed to solve a crisis in UK air capacity after decades of dithering and delay.
The plan was selected as the best option in a report by the Airports Commission, which looked at how UK air capacity should expand.
The Commission ruled out options like "Boris Island" in the Thames Estuary then settled on three final options. They were expanding Gatwick and a rival Heathrow plan, along with the current favourite.
Will it cost any public money?
Heathrow bosses say no.
But Tory MPs had warned the taxpayer could pick up the tab.
And green groups say the knock-on costs will be felt in changes to the environment.
Will holidaymakers foot the bill?
It's been suggested strongly that air fares could go up on flights departing from the new airport.
That's because airlines would be charged more to park their planes at Heathrow.