  The International Axion Observatory (IAXO) is a next generation axion
helioscope aiming at a sensitivity to the axion-photon coupling of a few
10^{-12} GeV^{-1}, i.e. 1-1.5 orders of magnitude beyond sensitivities achieved
by the currently most sensitive axion helioscope, the CERN Axion Solar
Telescope (CAST). Crucial factors in improving the sensitivity for IAXO are the
increase of the magnetic field volume together with the extensive use of x-ray
focusing optics and low background detectors, innovations already successfully
tested at CAST. Electron-coupled axions invoked to explain the white dwarf
cooling, relic axions, and a large variety of more generic axion-like particles
(ALPs) along with other novel excitations at the low-energy frontier of
elementary particle physics could provide additional physics motivation for
IAXO.
