In 1960, Yoichiro Nambu imports into elementary particle physics a concept he had observed working elegantly in superconductivity theory: spontaneous symmetry breaking. The central idea is counterintuitive but powerful: the fundamental laws governing a physical system can possess perfect symmetry, and yet the system's true minimum-energy state — its ground state or "vacuum" — may not respect that symmetry, spontaneously "choosing" one particular configuration among the many equally valid possibilities the symmetry would allow. Nambu, together with Giovanni Jona-Lasinio, develops a model in which this idea explains the origin of the mass of certain subatomic particles from interactions that, in themselves, do not distinguish between massive and massless particles. Working later with Jeffrey Goldstone, Abdus Salam, and Steven Weinberg, Nambu helps formalize what is today known as Goldstone's theorem: whenever a continuous symmetry is spontaneously broken, a massless associated particle must appear, called a Nambu-Goldstone boson. This conceptual framework would turn out to be the foundation on which, a few years later, Peter Higgs, François Englert, and Robert Brout would build the mechanism bearing Higgs's name, in which spontaneous symmetry breaking of the Higgs field gives mass to the elementary particles of the standard model. Nambu's contribution was, therefore, less a specific experimental discovery than the introduction of a deep organizing principle — taken by direct analogy from the Ginzburg-Landau theory of superconductivity — that would become one of the most fertile conceptual tools of all 20th-century theoretical physics, present today in virtually every corner of quantum field theory.