  The recent discovery of a new class of recurrent and fast X-ray transient
sources, the Supergiant Fast X-ray Transients, poses interesting questions on
the possible mechanisms responsible for their transient X-ray emission. The
association with blue supergiants, the spectral properties similar to those of
accreting pulsars and the detection, in a few cases, of X-ray pulsations,
confirm that these transients are High Mass X-ray Binaries. I review the
different mechanisms proposed to explain their transient outbursts and the link
to persistent wind accretors. I discuss the different models in light of the
new observational results coming from an on-going monitoring campaign of four
Supergiant Fast X-ray Transients with Swift.
