This review by T. Schäfer and E.V. Shuryak provides a comprehensive overview of the theory and phenomenology of instantons in Quantum Chromodynamics (QCD). The authors begin with a general introduction, followed by a pedagogical discussion of semi-classical methods in quantum mechanics and field theory. The main body of the review focuses on the understanding of the instanton liquid in QCD and its role in generating the spectrum of light hadrons. They also explore the properties of instantons at finite temperature and their potential role in the chiral phase transition. Additionally, the review covers the application of instantons in other models, such as low-dimensional sigma models, electroweak theory, and supersymmetric QCD. The review is structured into several sections, including an introduction to hadronic structure and the QCD vacuum, scales of non-perturbative QCD, the structure of the QCD vacuum, and the history of instantons. It discusses the discovery and early applications of instantons, the development of the instanton liquid model, and recent progress in understanding the instanton liquid at finite temperature and the chiral phase transition. The authors also address topics that are not discussed in detail, such as the direct manifestations of small-size instantons in high-energy baryon number violating reactions. The review emphasizes the importance of instantons in understanding the non-perturbative aspects of QCD, particularly in the context of hadronic structure and the chiral symmetry breaking. It highlights the role of instantons in generating the spectrum of light hadrons and their implications for the chiral phase transition. The review also discusses the technical developments and numerical simulations that have advanced the field, including the construction of self-consistent interacting instanton ensembles and the comparison of lattice calculations with the instanton model.This review by T. Schäfer and E.V. Shuryak provides a comprehensive overview of the theory and phenomenology of instantons in Quantum Chromodynamics (QCD). The authors begin with a general introduction, followed by a pedagogical discussion of semi-classical methods in quantum mechanics and field theory. The main body of the review focuses on the understanding of the instanton liquid in QCD and its role in generating the spectrum of light hadrons. They also explore the properties of instantons at finite temperature and their potential role in the chiral phase transition. Additionally, the review covers the application of instantons in other models, such as low-dimensional sigma models, electroweak theory, and supersymmetric QCD. The review is structured into several sections, including an introduction to hadronic structure and the QCD vacuum, scales of non-perturbative QCD, the structure of the QCD vacuum, and the history of instantons. It discusses the discovery and early applications of instantons, the development of the instanton liquid model, and recent progress in understanding the instanton liquid at finite temperature and the chiral phase transition. The authors also address topics that are not discussed in detail, such as the direct manifestations of small-size instantons in high-energy baryon number violating reactions. The review emphasizes the importance of instantons in understanding the non-perturbative aspects of QCD, particularly in the context of hadronic structure and the chiral symmetry breaking. It highlights the role of instantons in generating the spectrum of light hadrons and their implications for the chiral phase transition. The review also discusses the technical developments and numerical simulations that have advanced the field, including the construction of self-consistent interacting instanton ensembles and the comparison of lattice calculations with the instanton model.