APEX is an acronym for the Atacama Pathfinder EXperiment, a single 12-m radio-wavelength antenna operated by the Max-Planck-Institut fur Radioastronomie (MPIfR) in conjunction with the European Southern Observatory (ESO) and the Onsala Space Observatory (OSO). APEX operates at submillimetre radio wavelengths. It has been operating since 2005 at the Chajnantor site in Chile where ALMA is being constructed.

As its name suggests, APEX is a precursor instrument, in this case to ALMA, and is actually a modified prototype ALMA antenna. At the time of construction, there were no submillimetre telescopes in the Southern Hemisphere, meaning that the Southern sky is largely unexplored at these wavelengths. APEX was conceived to perform low-resolution wide-field observations that serve as a precursor to follow-up, high-resolution observations that will be possible once ALMA is completed.

The high frequency observations that APEX is primed for, between 211 and 500 GHz, are suitable for looking at molecular spectral lines and warm dust in regions as diverse as star-forming regions and galactic nuclei, from our Milky Way to galaxies at high redshift.