  One of the major achievements of Swift is the discovery of the erratic X-ray
flares harboring nearly half of gamma-ray bursts (GRBs), both for long-duration
and short-duration categories, and both for traditional hard GRBs and soft
X-ray flashes (XRFs). Here I review the arguments in support of the suggestion
that they are powered by reactivation of the GRB central engine, and that the
emission site is typically ``internal'', i.e. at a distance within the forward
shock front. The curvature effect that characterizes the decaying lightcurve
slope during the fading phase of the flares provides an important clue. I will
then discuss several suggestions to re-start the GRB central engine and comment
on how future observations may help to unveil the physical origin of X-ray
flares.
