The efficiency of a thermoelectric material is typically described using the dimensionless figure of merit 𝑍𝑇: 𝑍𝑇 = 𝑆 2 𝜎𝑇 𝜅 latt + 𝜅 el where 𝑆 is the Seebeck coefficient, 𝜎 is the electrical conductivity, the product 𝑆 2 𝜎 is the power factor, and 𝜅 latt and 𝜅 el are the lattice (phonon) and electronic contributions to the thermal conductivity. 𝑆, 𝜎 and 𝜅 el depend on the electronic structure and the carrier concentration, and are interdependent such that the 𝑍𝑇 at a target operating temperature is typically maximised in heavily-doped semiconductors. The bulk of the 𝜅 latt in semiconductors is from heat transport through phonons. This is largely independent of the electronic structure, and can be minimised to improve the 𝑍𝑇 using a variety of strategies spanning chemical doping and alloying, to including nanoscale phase impurities, to mesoscale structuring. The majority of the current flagship TEs are heavy p-block chalcogenides including PbTe, SnSe and the current industry standard Bi2Te3. These systems have in common narrow bandgaps and favourable electronic transport, together with large intrinsic phonon anharmonicity leading to low 𝜅 latt . However, the constituent elements are rare and/or environmentally hazardous, which prohibits the large-scale adoption of TEGs based on these materials.