The muon was discovered accidentally in 1936-1937 in the hunt for Yukawa's pion. At the time it seemed to serve no purpose in the scheme of particle physics. I.I. Rabi asked, "Who ordered that?", when he was told of it. It proved to be only the first of a soon to be huge number of anomalous particles that forced many new theories to be developed. Physicists learned that many of the particles previously considered fundamental, such as the proton and neutron, are actually composite objects. However, when the dust finally settled and the Standard Model was born, the muon retained it's status as fundamental, as the charged lepton in the second generation of elementary particles.

[Ed. note: Corrected muon discovery date 10/19/2001]