 The phase of the quantum wavefunction encodes internal degrees of freedom. With the requirement that a theory be invariant under local gauge transformations involving the phase of the wavefunction, Weyl’s ideas of 1918 found a successful home in quantum theory (see O’Raifeartaigh, 1997). Weyl’s new 1929 theory was a theory of electromagnetism coupled to matter. The history of gauge theory is surveyed briefly by Martin (2003), who highlights various issues surrounding gauge symmetry, in particular the status of the so-called “gauge principle”, first proposed by Weyl. The main steps in development of gauge theory are the Yang and Mills non-Abelian gauge theory of 1954, and the problems and solutions associated with the successful development of gauge theories for the short-range weak and strong interactions.