  The recent observations of ferromagnetic order in several two-dimensional
(2D) materials have generated an enormous interest in the physical mechanisms
underlying 2D magnetism. In the present prospective article we show that
Density Functional Theory (DFT) combined with either classical Monte Carlo
simulations or renormalized spin-wave theory can predict Curie temperatures for
ferromagnetic insulators that are in quantitative agreement with experiment.
The case of materials with in-plane anisotropy is then discussed and it is
argued that finite size effects may lead to observable magnetic order in
macroscopic samples even if long range magnetic order is forbidden by the
Mermin-Wagner theorem.
