  Magnetic reconnection is believed to be the driver of many explosive
phenomena in Astrophysics, from solar to gamma-ray flares in magnetars and in
the Crab nebula. However, reconnection rates from classic MHD models are far
too slow to explain such observations. Recently, it was realized that when a
current sheet gets sufficiently thin, the reconnection rate of the tearing
instability becomes "ideal", in the sense that the current sheet destabilizes
on the "macroscopic" Alfv\'enic timescales, regardless of the Lundquist number
of the plasma. Here we present 2D compressible MHD simulations in the
classical, Hall, and relativistic regimes. In particular, the onset of
secondary tearing instabilities is investigated within Hall-MHD for the first
time. In the frame of relativistic MHD, we summarize the main results from Del
Zanna et al. [1]: the relativistic tearing instability is found to be extremely
fast, with reconnection rates of the order of the inverse of the light crossing
time, as required to explain the high-energy explosive phenomena.
