Fe-Ni-S phases are accessory phases in the Earth’s mantle, but carry important geophysical and geochemical implications. Their physical and chemical properties differ greatly from the silicates and oxides. These include higher density, higher conductivity, lower melting temperatures and preferential partitioning of chalcophile and siderophile elements. This talk will summarize the work using both high-pressure experiments and thermodynamic calculations to constrain the melting temperatures and compositions of Fe-Ni-S phases in the Earth’s mantle. Based on these constraints, discussion will be made on the potential of Fe-Ni-S phases for deep carbon storage, primordial components and the cause for large low shear velocity provinces.