Space shuttle mission STS-134 will be Endeavour's last launch before it is taken out of service. Crew members include Commander Mark Kelly and astronaut Roberto Vittori of the European Space Agency.
During its 16-day mission, Endeavour and its crew will deliver the Alpha Magnetic Spectrometer (AMS), along with a variety of spare parts to the International Space Station.
The AMS is described as a 'state-of-the-art particle physics detector'. It was designed and built by a team comprising 56 institutes and 16 countries around the world. The spectrometer will be used, in the unique environment of space to advance our understanding of the universe and its origins by looking for antimatter and dark matter.
Endeavor is NASA's fifth and final space shuttle. Construction began in 1987 and was complete in April 1991. It launched for the first time on May 7, 1992 and has travelled 116,372, 930 miles so far in its lifetime.
You can follow the progress of today's launch on the launch blog.
You can also watch the preparations and launch on NASA TV