John (Jean) Iliopoulos is a Greek physicist. He is the first to present the standard model of particle physics in a single reference. He is best known for his prediction of the fourth quark, alongside Sheldon Lee Glashow and Luciano Maiani (the “GIM mechanism”).

He demonstrated the cancellation of anomalies in the Standard Model and introduced the Fayet-Iliopoulos D-term. Today, he is an honorary member of the Laboratory of Theoretical Physics at the École Normale Supérieure in Paris. He is a pioneer of supersymmetry, the hypothetical symmetry that links fermions and bosons. He also studied certain aspects of the quantum theory of gravity, as well as the mathematical properties of theories that remain invariant under gauge transformations and are formulated in a space with non-commutative geometry.