Pyrrhotite is an iron sulphide mineral (Fe1?xS, where x = 0–0.125) with variable compositions, crystal structures, Fe/S contents, and magnetic properties. Pyrrhotite polytypes occur in reducing igneous, metamorphic, sedimentary, and extraterrestrial rocks. At ambient temperatures, monoclinic (4C) pyrrhotite (Fe7S8) is ferrimagnetic; the hexagonal 3C polytype with Fe7S8 composition is also ferrimagnetic, while more S-rich compositions (Fe9S10, Fe11S12) are antiferromagnetic. Monoclinic pyrrhotite has a Curie temperature of 325°C, it passes through the low-temperature Besnus magnetic transition at 34 K, it has low to medium coercivities, and can have uniaxial and/or triaxial anisotropy. The low-field magnetic susceptibility is a complex parameter for 4C pyrrhotite because of its dependence on particle size, applied field strength, and applied field frequency. The 3C pyrrhotite polytype lacks a low-temperature magnetic transition and has higher coercivities than 4C pyrrhotite. Despite being one of the earliest studied magnetic minerals, important magnetic parameters for 4C pyrrhotite remain unknown and much more work is needed to document the magnetic properties of other polytypes.