How can star formation be sustained?
  There is overwhelming evidence that the Milky Way has formed its stars at a
rel- atively constant rate throughout the Hubble time. This implies that its
stock of cold gas was not in place since the beginning but it has been acquired
slowly through gas accretion. The gas accretion must have been at low
metallicity in order to reconcile the metallicities observed in the disc with
the predictions of chemical evolution models. But how does this gas accretion
take place? I review the current evidence of gas accretion into the Milky Way
and similar galaxies through the infall of cold gas clouds and satellites. The
conclusion from these studies is that the infalling gas at high column
densities observed in HI emission is a least one order of magnitude below the
value required to sustain star formation, thus accretion must come from a
different channel. The likely reservoir for gas accretion is the cosmological
corona of virial-temperature gas in which every galaxy must be embedded. At the
interface between the disc and the corona the cold high-metallicity disc gas
and the hot coronal medium must mix efficiently and this mix- ing causes the
cooling and accretion of the lower corona. I show how this mechanism reproduces
the kinematics of the neutral extraplanar gas in the Milky Way and other nearby
galaxies and the ionised high-velocity clouds observed in HST spectra. I
conclude with the speculation that the loss in efficiency of the disc-corona
interaction is the ultimate cause for the evolution of disc galaxies towards
the red sequence.
