Instantons in QCD: A Review This review discusses the theory and phenomenology of instantons in Quantum Chromodynamics (QCD). Instantons are classical solutions to the Euclidean equations of motion in QCD, characterized by a topological charge and associated with tunneling events between degenerate classical vacua. They play a crucial role in generating the spectrum of light hadrons and are important for understanding the structure of the QCD vacuum and the chiral phase transition. The review begins with an introduction to the motivation and physics outlook, covering topics such as hadronic structure, scales of non-perturbative QCD, the structure of the QCD vacuum, and QCD at finite temperature. It then provides a pedagogical introduction to semi-classical methods in quantum mechanics and field theory, focusing on tunneling in quantum mechanics, the double well potential, and the tunneling amplitude in Yang-Mills theory. The main part of the review discusses the role of instantons in QCD, including their properties at finite temperature, their contribution to the chiral phase transition, and their role in generating the spectrum of light hadrons. It also covers the instanton liquid model, the interaction of instantons with light quarks, and the connection between the structure of the vacuum, hadronic correlation functions, and hadronic structure. The review also discusses the instanton ensemble, including the mean field approximation, the quark condensate, and the effective interaction between quarks. It explores the Dirac eigenvalues and chiral symmetry breaking, as well as the spin-dependent interactions induced by instantons. The review then turns to the role of instantons in other theories, such as low-dimensional sigma models, electroweak theory, and supersymmetric QCD. It concludes with a summary and discussion of the main findings, emphasizing the importance of instantons in understanding the structure of the QCD vacuum and the hadronic spectrum. The review highlights the significance of instantons in explaining the chiral symmetry breaking and the role of the instanton liquid in generating the spectrum of light hadrons. It also discusses the impact of instantons on the chiral phase transition and their role in the structure of the QCD vacuum at finite temperature. The review provides a comprehensive overview of the current understanding of instantons in QCD and their relevance to the broader field of non-perturbative QCD.Instantons in QCD: A Review This review discusses the theory and phenomenology of instantons in Quantum Chromodynamics (QCD). Instantons are classical solutions to the Euclidean equations of motion in QCD, characterized by a topological charge and associated with tunneling events between degenerate classical vacua. They play a crucial role in generating the spectrum of light hadrons and are important for understanding the structure of the QCD vacuum and the chiral phase transition. The review begins with an introduction to the motivation and physics outlook, covering topics such as hadronic structure, scales of non-perturbative QCD, the structure of the QCD vacuum, and QCD at finite temperature. It then provides a pedagogical introduction to semi-classical methods in quantum mechanics and field theory, focusing on tunneling in quantum mechanics, the double well potential, and the tunneling amplitude in Yang-Mills theory. The main part of the review discusses the role of instantons in QCD, including their properties at finite temperature, their contribution to the chiral phase transition, and their role in generating the spectrum of light hadrons. It also covers the instanton liquid model, the interaction of instantons with light quarks, and the connection between the structure of the vacuum, hadronic correlation functions, and hadronic structure. The review also discusses the instanton ensemble, including the mean field approximation, the quark condensate, and the effective interaction between quarks. It explores the Dirac eigenvalues and chiral symmetry breaking, as well as the spin-dependent interactions induced by instantons. The review then turns to the role of instantons in other theories, such as low-dimensional sigma models, electroweak theory, and supersymmetric QCD. It concludes with a summary and discussion of the main findings, emphasizing the importance of instantons in understanding the structure of the QCD vacuum and the hadronic spectrum. The review highlights the significance of instantons in explaining the chiral symmetry breaking and the role of the instanton liquid in generating the spectrum of light hadrons. It also discusses the impact of instantons on the chiral phase transition and their role in the structure of the QCD vacuum at finite temperature. The review provides a comprehensive overview of the current understanding of instantons in QCD and their relevance to the broader field of non-perturbative QCD.