This review discusses the Nambu-Jona-Lasinio (NJL) approach to the dynamical breaking of chiral symmetry in Quantum Chromodynamics (QCD). After an overview of non-perturbative aspects of QCD, the NJL model is introduced as a low-energy effective theory of QCD. The model unifies the collective nature of hadrons and the constituent quark model, and can describe various aspects of QCD related to chiral symmetry breaking and the axial anomaly. Part I covers topics such as the vacuum structure of QCD, mass spectra, flavor mixing, and the validity of chiral perturbation. It also discusses the interplay between the axial anomaly and current-quark masses, and the problem of elusive scalar mesons. Part II applies the NJL model to systems at finite temperature and density, examining phenomena associated with chiral symmetry restoration. The model is used to study quark condensates, meson properties, and fluctuation phenomena near the critical temperature. The review highlights the importance of the NJL model in understanding the non-perturbative aspects of QCD and its applications to hadronic systems.This review discusses the Nambu-Jona-Lasinio (NJL) approach to the dynamical breaking of chiral symmetry in Quantum Chromodynamics (QCD). After an overview of non-perturbative aspects of QCD, the NJL model is introduced as a low-energy effective theory of QCD. The model unifies the collective nature of hadrons and the constituent quark model, and can describe various aspects of QCD related to chiral symmetry breaking and the axial anomaly. Part I covers topics such as the vacuum structure of QCD, mass spectra, flavor mixing, and the validity of chiral perturbation. It also discusses the interplay between the axial anomaly and current-quark masses, and the problem of elusive scalar mesons. Part II applies the NJL model to systems at finite temperature and density, examining phenomena associated with chiral symmetry restoration. The model is used to study quark condensates, meson properties, and fluctuation phenomena near the critical temperature. The review highlights the importance of the NJL model in understanding the non-perturbative aspects of QCD and its applications to hadronic systems.