  The observed properties of stars and especially the neutrino signal of the
supernova 1987A provide an upper limit to the axion mass, while the age and
expansion rate of the universe provide a lower limit. There remains a "window
of opportunity" 10^{-5} eV < m_a < 10^{-2} eV, with large uncertainties on
either side, where axions could still exist and where they would provide a
significant fraction or all of the cosmic dark matter. The current status of
this axion window is reviewed.
