  Magnetic reconnection is thought to be the dynamical mechanism underlying
many explosive phenomena observed both in space and in the laboratory, though
the question of how fast magnetic reconnection is triggered in such high
Lundquist ($S$) number plasmas has remained elusive. It has been well
established that reconnection can develop over timescales faster than those
predicted traditionally once kinetic scales are reached. It has also been shown
that, within the framework of resistive Magnetohydrodynamics (MHD), fast
reconnection is achieved for thin enough sheets via the onset of the so-called
plasmoid instability. The latter was discovered in studies specifically devoted
to the Sweet-Parker current sheet, either as an initial condition or an
apparent transient state developing in nonlinear studies. On the other hand, a
fast tearing instability can grow on an ideal, i.e., $S$-independent, timescale
(dubbed "ideal" tearing) within current sheets whose aspect ratio scales with
the macroscopic Lundquist number as $L/a\sim S^{1/3}$ -- much smaller than the
Sweet-Parker one -- suggesting a new way to approach to the initiation of fast
reconnection in collapsing current configurations. Here we present an overview
of what we have called "ideal" tearing in resistive MHD, and discuss how the
same reasoning can be extended to other plasma models commonly used that
include electron inertia and kinetic effects. We then discuss a scenario for
the onset of "ideal" fast reconnection via collapsing current sheets and
describe a quantitative model for the interpretation of the nonlinear evolution
of "ideally" unstable sheets in two dimensions.
