Star forming galaxies require a continuous supply of fresh gas to feed their
star formation. Such gas must come from the intergalactic environment where
most baryons are thought to reside. But how and in what form does this gas
accretion take place? Although some galaxies have been shown to be surrounded
by large reservoirs of cold gas in the form of extended disks and halos, direct
detections of accretion are difficult to obtain. The available estimates
consistently show that most of the gas accretion must be "hidden", i.e. not
observed directly. I review recent observational and theoretical results and
propose ways in which this gas accretion can occur.