Ellis' work is of importance to the study of elementary particles at colliders, such as the Fermilab Tevatron, and the CERN Large Hadron Collider. Ellis has contributed in a substantial way to the interpretation of experiments performed at high energy. Together with Douglas Ross and Tony Terrano he performed the first calculation of jet structure in e+e- annihilation which allowed precise determination of the strong coupling. In addition, with Guido Altarelli and Guido Martinelli he performed a calculation of lepton pair production which allow reconciliation of observed rates with theoretical calculations. He has also co-authored a number of widely read papers on the theory of heavy quark production. He is also co-author for the parton-level Monte Carlo program MCFM.