Centuries after the search began for the fundamental constituents that make up all the complexity and beauty of the everyday world, we have an astonishingly simple answer--it takes just six particles: the electron, the up and the down quarks, the gluon, the photon and the Higgs boson. Eleven additional particles suffice to describe all the esoteric phenomena studied by particle physicists [see box at right]. This is not speculation akin to the ancient Greeks' four elements of earth, air, water and fire. Rather it is a conclusion embodied in the most sophisticated mathematical theory of nature in history, the Standard Model of particle physics. Despite the word model in its name, the Standard Model is a comprehensive theory that identifies the basic particles and specifies how they interact. Everything that happens in our world (except for the effects of gravity) results from Standard Model particles interacting according to its rules and equations.