I will discuss our research on 2D magnetic materials and heterostructures. After a short introduction, I will present results on atomically thin multilayers of different magnetic semiconductors such as CrI3, CrCl3 (layered antiferromagnets), MnPS3 (antiferromagnetic within individual layers), and CrBr3 (ferromagnetic semiconductors). Using atomically thin, exfoliated crystals, we form tunnel barriers that enable magnetism to be probed by magnetotransport measurements. Examples of observed phenomena include: i) a giant tunneling magnetoresistance in CrI3; ii) a full characterization of the magnetic phase diagram of CrCl3 multilayers; iii) the observation of a spin-flop transition in MnPS3 persisting to the ultimate thickness of an individual monolayer, and iv) the demonstration that the tunneling magnetoresistance of ferromagnetic CrBr3 barriers depends on magnetic field and temperature only through the magnetization (from well above to well below the Curie temperature). We conclude that measurements of the temperature and magnetic field dependence of the tunneling magnetoresistance allow precise information about the magnetic state of atomically thin crystals to be obtained, something impossible to do with most conventional experimental techniques, not sufficiently sensitive when used on such a small amount of material.