APEX, the Atacama Pathfinder Experiment, is a 12 m diameter submillimetre telescope at 5100 m altitude in the Atacama Desert in northern Chile.

APEX was inaugurated in September 2005, and provides new opportunities for astronomers to use submillimetre observations to study the first galaxies and how stars and planets are formed.

The APEX project is a collaboration between Onsala Space Observatory, Max Planck Institut für Radioastronomie (Germany), and the European Southern Observatory to construct and operate a modified ALMA prototype antenna as a single dish on the high altitude site of Llano Chajnantor. The telescope was supplied by VERTEX Antennentechnik in Germany. Onsala Space Observatory, through the Group for Advanced Receiver Development (GARD), provides facility heterodyne receivers for several frequency bands up to 1.3 THz.