  Magnetic reconnection is a fundamental process in a laboratory,
magnetospheric, solar and astrophysical plasma, whereby magnetic energy is
converted into heat, bulk kinetic energy and fast particle energy. Its nature
in two dimensions is much better understood than in three dimensions (3D),
where its character is completely different and has many diverse aspects that
are currently being explored. Here we focus on the magnetohydrodynamics of 3D
reconnection in the plasma environment of the solar system, especially solar
flares.
  The theory of reconnection at null points, separators and quasi-separators is
described, together with accounts of numerical simulations and observations of
these three types of reconnection. The distinction between separator and
quasi-separator reconnection is a theoretical one that is unimportant for the
observations of energy release. A new paradigm for solar flares, in which 3D
reconnection plays a central role, is proposed.
