ATHENA, one of the three approved experiments at the new facility for low e
nergy antiprotons (AD) at CERN, has the primary goal to test CPT invariance
by comparing the atomic energy levels of antihydrogen to those of hydrogen
. The extended experimental program also contains studies on differences in
gravitational acceleration of antimatter and matter. The production of ant
ihydrogen atoms and their spectral response to laser light will be monitore
d by a sophisticated detector for the end products of antiproton and positr
on annihilations.