In 2002, Iliopoulos was the first recipient of the Aristeio prize, which has been instituted to recognize Greeks who have made significant contributions towards furthering their chosen fields of science.

[5] In 2007 Iliopoulos and Maiani received the Dirac Medal of the ICTP "(f)or their work on the physics of the charm quark, a major contribution to the birth of the Standard Model, the modern theory of Elementary Particles."

In 1970, in collaboration with Sheldon Glashow and Luciano Maïani, he introduced the so-called "GIM mechanism" (named after the three authors) which is an essential element of the theory of fundamental interactions known as the "Standard Model ".

In 1972, in collaboration with Claude Bouchiat and Philippe Meyer,[8] he demonstrated that the mathematical coherence of the Standard Model requires symmetry between the elementary constituents of matter, namely quarks (which form hadrons such as proton and neutron) and leptons (such as electron, muon and neutrinos).

He showed that it has remarkable convergence properties and, in collaboration with Pierre Fayet,[9] he proposed a mechanism that leads to its spontaneous breakage.