The Atacama Pathfinder Experiment telescope is operated by ESO in collaboration with the Max Planck Institute for Radio Astronomy in Bonn, Germany and the Onsala Space Observatory in Onsala, Sweden. It is a -diameter telescope operating at millimetre and submillimetre wavelengths, the largest of its kind in the southern hemisphere. APEX is a precursor of ALMA (the Atacama Large Millimeter Array), an astronomical interferometer which ESO and its international partners is building on the Chajnantor plateau. APEX is based on a prototype ALMA antenna that is modified to be operated as single dish radio telescope.