Arthur’s development of the trace formula for reductive groups is a monumental mathematical achievement. It generalizes the Selberg trace formula for SL(2) from 1956. In his work, Arthur introduced many major tools in non-commutative harmonic analysis on general reductive groups. Building on the work of Langlands, Shelstad, Kottwitz, Waldspurger and others, Arthur obtained the trace formula in stable form. Using the Fundamental Lemma proved by Ngo, Arthur's work culminated in his description, as envisioned by Langlands, of the structure of automorphic representations of classical groups (symplectic groups and quasi-split special orthogonal groups). Some of the highlights are the functoriality associated to the standard representation, the multiplicity formulas in the discrete spectrum, the classification of the expected counter-examples to the generalized Ramanujan conjecture, and the description of local L-packets and global A-packets. Arthur's work had an enormous impact. For example, it had been a central tool in Lafforgue's proof of the Langlands correspondence for function fields. Recently it has been used by Clozel, Harris, Taylor and others in constructing Galois representations associated to automorphic forms via p-adic methods. Arthur's ideas, achievements and the techniques he introduced will have many more deep applications in the theory of automorphic representations, and the study of locally symmetric spaces. Arthur's work is a mathematical landmark that will inspire future generations of mathematicians.