Keith Ellis has made seminal contributions to the theory of the strong interactions, quantum chromodynamics (QCD). He performed many of the key calculations that led to the acceptance of QCD as the correct theory. These include the first quantitatively reliable predictions of fundamental processes observed in electron–positron and proton– (anti-)proton colliders. These results were essential for the analysis of the properties of gluons and the discovery of the top quark. He continues to play a leading role in the development of new techniques for precision calculation in quantum field theory.