Atlantis lifts off on NASA's 135th and final shuttle mission.
Update at 11:50 a.m. ET: The shuttle was visible for 42 seconds before disappearing into the clouds.
NASA waived its own weather rules to allow the liftoff to go forward.
In the end, though, the countdown was delayed not by the weather but by the need to verify that the launch pad support equipment was retracted all the way.
The crew will deliver a year's worth of critical supplies to the International Space Station and return with as much trash as possible. Atlantis is scheduled to come home on June 20 after 12 days in orbit.
Before taking flight, Commander Christopher Ferguson saluted all those who contributed over the years to the shuttle program.
"The shuttle is always going to be a reflection of what a great nation can do when it dares to be bold and commits to follow through," he said. "We're not ending the journey today … we're completing a chapter of a journey that will never end."
Update at 11:40 a.m. ET: NASA: 'Flawless.'
Update at 11:38 a.m. ET: The external fuel tank has fallen away.
Update at 11:37 a.m. ET: Main engine cutoff.
Update at 11:35 a.m. ET: Mission control says Atlantis ist 315 miles down-range, traveling at 6500 miles an hour on a track "straight as an arrow" to the International Space Station.
Update at 11:33 a.m. ET: The flight will last 12 days. Weather permitting, Atlantis will return to the Kennedy Space Ceneter, where it will end up on permanent display.