Thermoelectric materials allow for the interconversion of heat and electricity. Rapid progress has been made in materials performance but there is a continuing need for the exploration of new materials and disruptive thermoelectric effects. Metal phosphides are currently gaining increasing attention as potential thermoelectric materials. Structural complexity often leads to low thermal conductivity despite the low atomic mass of P, which underpins promising thermoelectric figures of merit, zT. Despite the ability of phosphorus to form strong covalent bonds, including polyanion bonding, a key challenge remains to embed better electrical properties and large power factors. A highest zT = 0.9 has been observed in n-type Cd3P2, whilst promising performance under applied fields has been observed in topological materials, e.g. in the Dirac metal TaP. In this review, we summarise recent materials developments and analyse some of the underlying thermoelectric parameters of metal phosphide materials.