This paper reviews the Nambu-Jona-Lasinio (NJL) model as a low-energy effective theory of Quantum Chromodynamics (QCD), focusing on the dynamical breaking of chiral symmetry. The authors provide a comprehensive overview of the non-perturbative aspects of QCD, including the vacuum structure, mass spectra, and coupling constants of hadrons. They discuss various phenomena related to the dynamical and explicit breaking of chiral symmetry and the axial anomaly, such as flavor mixing in mesons, OZI rule violation in baryons, and the validity of chiral perturbation theory in QCD. The NJL model is then applied to systems at finite temperature and density, relevant to the early universe, neutron stars, and ultra-relativistic heavy ion collisions. The paper explores the restoration of chiral symmetry in these systems, the properties of mesons at finite temperature and density, and the experimental implications. It also delves into the existence of soft modes and the quark-number susceptibility in the vector channel. The authors conclude by discussing the significance of the NJL model in understanding the non-perturbative dynamics of QCD and its applications to various physical phenomena.This paper reviews the Nambu-Jona-Lasinio (NJL) model as a low-energy effective theory of Quantum Chromodynamics (QCD), focusing on the dynamical breaking of chiral symmetry. The authors provide a comprehensive overview of the non-perturbative aspects of QCD, including the vacuum structure, mass spectra, and coupling constants of hadrons. They discuss various phenomena related to the dynamical and explicit breaking of chiral symmetry and the axial anomaly, such as flavor mixing in mesons, OZI rule violation in baryons, and the validity of chiral perturbation theory in QCD. The NJL model is then applied to systems at finite temperature and density, relevant to the early universe, neutron stars, and ultra-relativistic heavy ion collisions. The paper explores the restoration of chiral symmetry in these systems, the properties of mesons at finite temperature and density, and the experimental implications. It also delves into the existence of soft modes and the quark-number susceptibility in the vector channel. The authors conclude by discussing the significance of the NJL model in understanding the non-perturbative dynamics of QCD and its applications to various physical phenomena.