Roger Powell has pioneered the development of quantitative approaches to metamorphic rocks by the generation of thermodynamic models for minerals and fluids, and writing the software that implements them. The programs comprise a self-consistent thermodynamic database which describes equilibria among the multi-component mineral phases important in rocks and with full propagation of errors and are among the most highly cited works in the geosciences. He has used these models to make fundamental advances in our understanding of metamorphic and igneous processes across a wide range of geological environments from the crust to deep mantle.