The crew are led out by pilots James Kelly and commander Eileen Collins.
The Discovery blasts into space on Tuesday in Cape Canaveral, Florida, USA.
"Three, two, one - and lift-off!"
The shuttle is taking food to the International Space Station (ISS).
The 12-day mission will be the first by a space shuttle since the Columbia orbiter broke up killing all seven astronauts on board in February 2003.
Some 150,000 people weathered a really hot early morning to watch the launch.
The launch date for Discovery was delayed by 13 days after a problem was detected with the fuel sensor.
Discovery is due to return to Earth on the morning of 7 August.
President George W Bush's wife, Laura, and brother the Florida Governor were among those watching.