  The discovery of ferromagnetic order in monolayer 2D crystals has opened a
new venue in the field of two dimensional (2D) materials. 2D magnets are not
only interesting on their own, but their integration in van der Waals
heterostructures allows for the observation of new and exotic effects in the
ultrathin limit. The family of Chromium trihalides, CrI$_3$, CrBr$_3$ and
CrCl$_3$, is, so far, the most studied among magnetic 2D crystals. In this
mini-review, we provide a perspective of the state of the art of the
theoretical understanding of magnetic 2D trihalides, most of which will also be
relevant for other 2D magnets, such as vanadium trihalides. We discuss both the
well-established facts, such as the origin of the magnetic moment and magnetic
anisotropy and address as well open issues such as the nature of the
anisotropic spin couplings and the magnitude of the magnon gap. Recent
theoretical predictions on Moir\' e magnets and magnetic skyrmions are also
discussed. Finally, we give some prospects about the future interest of these
materials and possible device applications.
