  The years of 1960-1965 were a remarkable period for Yoichiro Nambu. Starting
with a reformulation of BCS theory with emphasis on gauge invariance, he
recognized the realization of spontaneous chiral symmetry breaking in particle
physics as is evidenced by the Goldberger-Treiman relation. A concrete model of
Nambu and Jona-Lasinio illustrated the essence of the Nambu-Goldstone theorem
and the idea of soft pions. After the proposal of the quark model by Gell-Mann,
he together with Han constructed an alternative model of integrally charged
quarks with possible non-Abelian gluons. All those remarkable works were
performed during the years 1960-1965. Here I briefly review those works
following the original papers of Nambu chronologically, together with a brief
introduction to a formulation of Neother's theorem and Ward-Takahashi
identities using path integrals. This article is mostly based on a lecture
given at the Nambu Memorial Symposium held at Osaka City University in
September 2015, where Nambu started his professional career.
