"Here's what we know: we are going to launch American astronauts on American rockets from American soil," says Jim Bridenstine.
He adds that today was still an "important milestone".
"We did a wet dress rehearsal - we haven't done a wet dress rehearsal, in full gear, before.
"We get a lot of great data from doing what we just did. So today was a good day...and we've got a lot to look forward to."
With that, Nasa's live coverage of the event is over.
'The right decision was made'
Nasa administrator Jim Bridenstine is speaking now.
"It was a great day for Nasa, it was a great day for SpaceX," he says. "I think our teams worked together in a really impressive way making good decisions all along."
The issue today, he explains, was just too much electricity in the atmosphere. Though there wasn't a lightning storm, there was a concern that the launch itself might trigger lightning.
"We had the parameters set ahead of time and in the end, the right decision was made."
Bridenstine says he often gets asked about the pressure to launch.
"People say to me, with all the attention of the world on this launch, with all of the VIPs coming, are you gonna feel pressure on this launch? And I will tell you...under no circumstances should anybody feel pressure.
"If we are not ready to go we simply do not go. I am proud, so proud, of our teams working together to make the right decision."
The astronauts are back outside
Astronauts Bob Behnken and Doug Hurley are heading back downstairs to ground level.
They'll remain isolated in crew quarters until the next launch attempt on Saturday.
We're now waiting for remarks from Nasa Administrator Jim Bridenstine.
nasaCopyright: nasa
Hatch is back open
After some pressure checks, the hatch is open and the astronauts will shortly be able to leave their seats.
If the engineers find no issues with the craft, the crew arm and capsule will remain where they are, vertical, until the next attempt.
NasaCopyright: Nasa
The evolution of the spacesuit
NasaCopyright: Nasa
One of the most eye-catching elements of this historic launch is undoubtedly the new SpaceX suits.
There are only certain times that work with the space station's orbit. Remember, the station is travelling at over 17,000mph (27,000km/h).
That means that on different days, the time to reach the space station from the launch site varies, and the amount of fuel each trip needs also changes.
Saturday will be the next best option - with an approximately 19-hour journey to the ISS then.
Astronauts prepare to exit capsule
The crew access arm has swung back to meet the capsule as all of the fuel has been offloaded.
The same technicians who helped lock them in will now come open the hatch.
The astronauts' seats - which they've been strapped into for the last three hours or so - will rotate back so the crew can help them out.
NasaCopyright: Nasa
Why did they scrub the launch?
The crew says the launch was cancelled due to three violations of weather rules - all three of which were expected to clear 10 minutes after the scheduled lift-off time. But, the launch time had no wiggle room today.
They were: natural lightning, electric fields in the atmosphere and attached anvil clouds (clouds associated with thunderstorms).
"We could see some raindrops on the windows and just figured that whatever it was was too close to the launch pad at the time we needed it not to be," Hurley says.
"We appreciate that and understand that everybody's probably a little bit bummed out, it's just part of the deal. Everybody was ready today and we appreciate that and the ship was great. We'll do it again, I think, on Saturday."
NASACopyright: NASA
What happens now?
The crew will now go through a sequence to end launch proceedings. It should take about 40 minutes for the fuel to be offloaded. Once that happens, the crew access arm will swing back to the capsule and the astronauts can exit.
In three days, we'll go through this all again on 30 May at 15:22 local time.
There's a 60% chance of good weather at that time, but as always with Florida in the springtime, storms are a possibility.
An early decision is made to delay to Saturday the first US astronaut launch in the US for nine years.
Poor weather has forced SpaceX to call off the launch of Nasa astronauts Doug Hurley and Bob Behnken to the International Space Station (ISS).
The two men were due to go up from the Kennedy Space Center in one of the company's new Dragon capsules at 16:33 local time (21:33 BST).
But unfavourable atmospheric conditions prompted controllers to call a stop to flight preparations for the day.
A tropical storm is also forming out over the Atlantic.
This would have been very close to the path of the astronauts' spaceship as it made its ascent to orbit.
SpaceX and Nasa will now have to wait until the space station is back in the right part of the sky to attempt another launch.
The earliest this can be is Saturday. If that's no good, there would be a third opportunity on Sunday.
There is great interest in this launch. Not since the retirement of the shuttles in 2011 has America been able to launch its own astronauts into space - a big gap in which the US has had to rely on Russian Soyuz vehicles.
But Hurley's and Behnken's mission is about more than just pride.
Nasa is giving up its past practice of owning and operating the space systems it uses low-Earth orbit and intends in future simply to buy crew transport services from the private sector – much like a company might outsource its payroll or HR needs.
SpaceX is the first of these new service providers.
Nasa Administrator Jim Bridenstine believes the approach will save his agency money that can then be spent on missions to the Moon and Mars.
“We envision a future where low-Earth orbit is entirely commercialised, where Nasa is one customer of many customers, where we have numerous providers that are competing on cost, on innovation and safety," he said.
"We are proving out a business model that ultimately enable us to go to the Moon this time sustainably. In other words we’re going to go to the Moon to stay.”
BreakingLaunch aborted
The weather has led to today's launch being cancelled.
The next date is Saturday.
Weather not looking good
NasaCopyright: Nasa
A final decision has yet to be made, but the weather isn't looking good for a launch today.
The weather officers must consider both the weather in Florida at the launch site and down range, in case astronauts have to abort in flight and land in the ocean.
We'll get another update shortly.
What's an instantaneous launch window?
The mission has an instantaneous launch window. This means that if there is a delay of any kind, and the launch cannot proceed at 14:33 local time, the crew will have to wait for the backup date, Saturday, to launch.
The crew is looking into issues with electricity in the atmosphere at the moment.
We should get another weather update in a few minutes.
The president tours the centre
President Donald Trump, First Lady Melania Trump, Vice-President Mike Pence and his wife, Karen Pence, are touring the Kennedy Space Center ahead of the launch.
Senior advisers Jared Kushner and Ivanka Trump, the president's daughter, are also present with their children.
The president and vice-president were also seen signing an item that will go up with the astronauts.
Getty ImagesCopyright: Getty Images
Getty ImagesCopyright: Getty Images
Getty ImagesCopyright: Getty Images
Got questions? We have answers.
SpaceXCopyright: SpaceX
What is SpaceX? Why is a private company launching Nasa astronauts? Why is the launch so important?
We've answered the most common questions about this mission, who's involved and why it all matters.
The crew will now begin loading the propellant and the launch escape system will be armed, the launch director says.
Everyone is now told to stay in their seats as the crew access arm is moved away from the capsule.
The situation looks good, but of course, it all still depends on the weather.
How much rain will fall between now and lift-off is a key consideration. A little precipitation would be fine, and things seem to be moving in the right direction, Nasa says.
NasaCopyright: Nasa
'Bob and I are go for launch'
"SpaceX Dragon, Bob and I are go for launch," says Doug Hurley, strapped within the capsule.
The astronauts are ready, so the next step will be the go/no go determination at 45 minutes before launch.
NASACopyright: NASA
Who are the astronauts?
ReutersCopyright: Reuters
Astronauts Bob Behnken (left) and Doug HurleyImage caption: Astronauts Bob Behnken (left) and Doug Hurley
Today, two US astronauts will achieve a world first when they launch to the International Space Station aboard a spacecraft built by Elon Musk's SpaceX.
Doug Hurley and Bob Behnken are about to break a nine-year hiatus for Nasa, becoming the first astronauts to launch from US soil since the retirement of the space shuttle in 2011.
Live Reporting
Ritu Prasad, Jonathan Amos and Paul Rincon
All times stated are UK
Getty ImagesCopyright: Getty Images 
NasaCopyright: Nasa 
nasaCopyright: nasa 
NasaCopyright: Nasa 
NasaCopyright: Nasa 
NasaCopyright: Nasa 
NASACopyright: NASA 
NasaCopyright: Nasa 
Getty ImagesCopyright: Getty Images 
Getty ImagesCopyright: Getty Images 
Getty ImagesCopyright: Getty Images 
SpaceXCopyright: SpaceX 
NasaCopyright: Nasa 
NASACopyright: NASA 
ReutersCopyright: Reuters Astronauts Bob Behnken (left) and Doug HurleyImage caption: Astronauts Bob Behnken (left) and Doug Hurley
Latest PostThat's all from us
Our live coverage has now concluded.
The historic Nasa and SpaceX manned launch was delayed thanks to weather issues in Florida - namely, lightning concerns.
The crew will try again on Saturday 30 May, at 15:22 local time, when weather conditions will hopefully be better for lift-off.
You can read more here.
Thanks for joining us.
'An important milestone'
"Here's what we know: we are going to launch American astronauts on American rockets from American soil," says Jim Bridenstine.
He adds that today was still an "important milestone".
"We did a wet dress rehearsal - we haven't done a wet dress rehearsal, in full gear, before.
"We get a lot of great data from doing what we just did. So today was a good day...and we've got a lot to look forward to."
With that, Nasa's live coverage of the event is over.
'The right decision was made'
Nasa administrator Jim Bridenstine is speaking now.
"It was a great day for Nasa, it was a great day for SpaceX," he says. "I think our teams worked together in a really impressive way making good decisions all along."
The issue today, he explains, was just too much electricity in the atmosphere. Though there wasn't a lightning storm, there was a concern that the launch itself might trigger lightning.
"We had the parameters set ahead of time and in the end, the right decision was made."
Bridenstine says he often gets asked about the pressure to launch.
"People say to me, with all the attention of the world on this launch, with all of the VIPs coming, are you gonna feel pressure on this launch? And I will tell you...under no circumstances should anybody feel pressure.
"If we are not ready to go we simply do not go. I am proud, so proud, of our teams working together to make the right decision."
The astronauts are back outside
Astronauts Bob Behnken and Doug Hurley are heading back downstairs to ground level.
They'll remain isolated in crew quarters until the next launch attempt on Saturday.
We're now waiting for remarks from Nasa Administrator Jim Bridenstine.
Hatch is back open
After some pressure checks, the hatch is open and the astronauts will shortly be able to leave their seats.
If the engineers find no issues with the craft, the crew arm and capsule will remain where they are, vertical, until the next attempt.
The evolution of the spacesuit
One of the most eye-catching elements of this historic launch is undoubtedly the new SpaceX suits.
So what's behind the redesign?
We took a look at the futuristic flight suits - and their predecessors - here.
Why can't they launch tomorrow?
There are only certain times that work with the space station's orbit. Remember, the station is travelling at over 17,000mph (27,000km/h).
That means that on different days, the time to reach the space station from the launch site varies, and the amount of fuel each trip needs also changes.
Saturday will be the next best option - with an approximately 19-hour journey to the ISS then.
Astronauts prepare to exit capsule
The crew access arm has swung back to meet the capsule as all of the fuel has been offloaded.
The same technicians who helped lock them in will now come open the hatch.
The astronauts' seats - which they've been strapped into for the last three hours or so - will rotate back so the crew can help them out.
Why did they scrub the launch?
The crew says the launch was cancelled due to three violations of weather rules - all three of which were expected to clear 10 minutes after the scheduled lift-off time. But, the launch time had no wiggle room today.
They were: natural lightning, electric fields in the atmosphere and attached anvil clouds (clouds associated with thunderstorms).
"We could see some raindrops on the windows and just figured that whatever it was was too close to the launch pad at the time we needed it not to be," Hurley says.
"We appreciate that and understand that everybody's probably a little bit bummed out, it's just part of the deal. Everybody was ready today and we appreciate that and the ship was great. We'll do it again, I think, on Saturday."
What happens now?
The crew will now go through a sequence to end launch proceedings. It should take about 40 minutes for the fuel to be offloaded. Once that happens, the crew access arm will swing back to the capsule and the astronauts can exit.
In three days, we'll go through this all again on 30 May at 15:22 local time.
There's a 60% chance of good weather at that time, but as always with Florida in the springtime, storms are a possibility.
Big day for Nasa and SpaceX stymied by weather
Jonathan Amos
Science correspondent, BBC News
An early decision is made to delay to Saturday the first US astronaut launch in the US for nine years.
Poor weather has forced SpaceX to call off the launch of Nasa astronauts Doug Hurley and Bob Behnken to the International Space Station (ISS).
The two men were due to go up from the Kennedy Space Center in one of the company's new Dragon capsules at 16:33 local time (21:33 BST).
But unfavourable atmospheric conditions prompted controllers to call a stop to flight preparations for the day.
A tropical storm is also forming out over the Atlantic.
This would have been very close to the path of the astronauts' spaceship as it made its ascent to orbit.
SpaceX and Nasa will now have to wait until the space station is back in the right part of the sky to attempt another launch.
The earliest this can be is Saturday. If that's no good, there would be a third opportunity on Sunday.
There is great interest in this launch. Not since the retirement of the shuttles in 2011 has America been able to launch its own astronauts into space - a big gap in which the US has had to rely on Russian Soyuz vehicles.
But Hurley's and Behnken's mission is about more than just pride.
Nasa is giving up its past practice of owning and operating the space systems it uses low-Earth orbit and intends in future simply to buy crew transport services from the private sector – much like a company might outsource its payroll or HR needs.
SpaceX is the first of these new service providers.
Nasa Administrator Jim Bridenstine believes the approach will save his agency money that can then be spent on missions to the Moon and Mars.
“We envision a future where low-Earth orbit is entirely commercialised, where Nasa is one customer of many customers, where we have numerous providers that are competing on cost, on innovation and safety," he said.
"We are proving out a business model that ultimately enable us to go to the Moon this time sustainably. In other words we’re going to go to the Moon to stay.”
BreakingLaunch aborted
The weather has led to today's launch being cancelled.
The next date is Saturday.
Weather not looking good
A final decision has yet to be made, but the weather isn't looking good for a launch today.
The weather officers must consider both the weather in Florida at the launch site and down range, in case astronauts have to abort in flight and land in the ocean.
We'll get another update shortly.
What's an instantaneous launch window?
The mission has an instantaneous launch window. This means that if there is a delay of any kind, and the launch cannot proceed at 14:33 local time, the crew will have to wait for the backup date, Saturday, to launch.
The crew is looking into issues with electricity in the atmosphere at the moment.
We should get another weather update in a few minutes.
The president tours the centre
President Donald Trump, First Lady Melania Trump, Vice-President Mike Pence and his wife, Karen Pence, are touring the Kennedy Space Center ahead of the launch.
Senior advisers Jared Kushner and Ivanka Trump, the president's daughter, are also present with their children.
The president and vice-president were also seen signing an item that will go up with the astronauts.
Got questions? We have answers.
What is SpaceX? Why is a private company launching Nasa astronauts? Why is the launch so important?
We've answered the most common questions about this mission, who's involved and why it all matters.
Read the full story here.
Propellant loading to begin
The crew will now begin loading the propellant and the launch escape system will be armed, the launch director says.
Everyone is now told to stay in their seats as the crew access arm is moved away from the capsule.
The situation looks good, but of course, it all still depends on the weather.
How much rain will fall between now and lift-off is a key consideration. A little precipitation would be fine, and things seem to be moving in the right direction, Nasa says.
'Bob and I are go for launch'
"SpaceX Dragon, Bob and I are go for launch," says Doug Hurley, strapped within the capsule.
The astronauts are ready, so the next step will be the go/no go determination at 45 minutes before launch.
Who are the astronauts?
Today, two US astronauts will achieve a world first when they launch to the International Space Station aboard a spacecraft built by Elon Musk's SpaceX.
Doug Hurley and Bob Behnken are about to break a nine-year hiatus for Nasa, becoming the first astronauts to launch from US soil since the retirement of the space shuttle in 2011.
So who are they?
Check out our profiles of these two history-making astronauts here.
What's happening now?
Things are moving about 25 minutes ahead of schedule, according to Nasa.
Weather operators are completing a final wind speed check, which will be used in the final go/no go determination.
Various teams in Nasa and SpaceX will also be conducting more checks, making sure conditions are good with the capsule, crew and the space station.
So far, ISS conditions are good to go, but it remains to be seen if the weather in Florida cooperates.
We're around an hour ahead of lift-off time, which is still slated for 16:33 local time.