at scale. Given that chalcogenides often show the required balance of favourable electrical properties and low κ latt , much research has focused on alternative compositions. The electrical properties required for a high-performance TE are similar to those needed for many other optoelectronic applications, and the Sn and Ge monochalcogenides SnS/SnSe and GeS/GeSe have both been widely explored as absorber materials for thin-film solar cells. SnSe has been identified as an extremely promising TE material with an ultra-high bulk ZT max ≈ 2.6 at 923 K, good ZT over a wide range of 300 -773 K, and an even higher ZT max ≈ 3.1 at 773 K in polycrystalline samples. SnS shows very similar structural chemistry to SnSe, and is advantageous due to the higher abundance and lower toxicity of S, but has yet to demonstrate comparable TE performance. The Ge chalcogenides exhibit similar structures and optoelectronic properties to the Sn analogues, but their TE performance is less well characterised. However, Ge is 1000× more abundant than Te and less toxic than Pb, making GeS and GeSe attractive as prospective TEs. A small number of computational studies have predicted the orthorhombic Pnma phase of GeSe to have a large ZT max = 2.5 at 800 K, competitive with SnSe, and Pnma GeS to attain ZT max = 0.73 at 700 K, close to the industry-standard ZT = 1. More recently, the R3m phase of GeSe was also reported to show a ZT > 1. In our previous work on the tin chalcogenides we developed a workflow for calculating the four parameters in the ZT equation (Eq. 1) from first principles, which was found to give accurate predictions for the well-characterised Pnma SnS and SnSe. In this work, we apply a similar approach to predict the electrical and thermal transport properties of the Pnma and R3m phases of GeS and GeSe, with good agreement to available experimental data. By considering both p-and n-type doping, we predict that n-type Pnma GeSe has the highest potential ZT max of 2.12 at 900 K, with a doping level of 6 × 10 19 cm -3 , which would make it a good partner for p-type SnSe in a thermoelectric couple. Moreover, the n-doped material is predicted to show the largest σ and lowest κ latt along the layered b direction, potentially yielding a much larger ZT > 3 if the growth direction can be controlled. We also predict that the R3m phases can potentially attain ZT > 1 at mid-to-high temperature, which experiments suggest can be improved by suppressing the κ latt by alloying.