In recent years, high energy gamma ray astronomy made some spectacular discoveries regarding observations on delayed arrival
of the more energetic photons from extreme astrophysical sources, such as Active Galactic Nuclei or Gamma Ray Bursts. There
are currently four such confirmed instances, with delays ranging from half a second to minutes. The astrophysics of the
cosmic sources in these cases is still a mystery. In fact, standard models for particle acceleration, seem not to be capable
of reproducing the events, and one needs modified mechanisms. This prompted speculations that these delays may also be
(partly) due to photon propagation in  non-trivial vacua of quantum gravity, with non-standard optical properties, which is
the result of
a space-time foam situation. The talk will review briefly both astrophysical and quantum gravity approaches to the subject
entailing modified dispersion relations for matter probes. As far as the latter are concerned, I will show how most of these
are ruled out from a plethora of other astrophysical observations, and  concentrate on one, inspired from string theory,
which appears to be compatible with the current astrophysical data, including the observed delays. This happens for
specifically stringy reasons, associated with a non commutative structure of space time at string scales, as well as the
fact that the string foam effects are not of universal strength among particle species, being dominant for high-energy
photons. The theoretical discussion will be non technical, aimed at non experts.