Ultrastrong coupling between light and matter has transitioned from a theoretical concept to experimental reality in the past decade. This regime of quantum light-matter interaction goes beyond weak and strong coupling, where the coupling strength becomes comparable to the transition frequencies in the system. Achieving weak and strong coupling has enabled greater control of quantum systems and applications such as lasers, quantum sensing, and quantum information processing. This review discusses the theory of quantum systems with ultrastrong coupling, including entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to important physical models. It also covers various experimental setups, such as superconducting circuits, organic molecules, semiconductor polaritons, and optomechanics, that have achieved ultrastrong coupling. The review discusses the potential applications of these achievements in physics and chemistry. The interaction between light and matter is typically described by the fine structure constant α ≈ 1/137. Purcell's discovery that the interaction of an emitter with light can be enhanced or suppressed by engineering its electromagnetic environment led to the development of cavity quantum electrodynamics (CQED). The fundamental importance of controlling the strength of light-matter coupling g led to the development of resonators with higher quality factors. In 1983, Haroche and colleagues achieved a coupling strength exceeding the system losses, entering the strong-coupling (SC) regime. In this regime, vacuum Rabi oscillations occur, while in the weak-coupling (WC) regime, energy is lost before exchange between light and matter. The SC regime was later achieved with single atoms and optical cavities. In 1992, the SC regime was demonstrated using quasi-2D electronic excitations in a semiconductor optical microcavity. The resulting system eigenstates are called cavity-polaritons. Following these experiments, CQED has been adapted using artificial atoms such as quantum dots and superconducting qubits. In a CQED setup, the dimensionless parameter quantifying the interaction is the ratio between the coupling strength g and the bare energy of the excitations. This quantity, the normalized coupling η, is proportional to a positive power of α. The first observations of the SC regime had η values smaller than 10⁻⁶ for atoms and 10⁻³ for Wannier excitons. Perturbation theory is adequate for these experiments. The important difference with the WC regime is that, being the coupling larger than the spectral width of the excitations, degenerate perturbation theory needs to be applied. It took over two decades after the observation of SC for the CQED community to begin investigating the possibility to access a regime with larger η. Two main paths were identified to reach such a regime. The first is to couple many dipoles to the same cavity mode, leading to an enhanced coupling which scales with the square root of the number of dipoles. The secondUltrastrong coupling between light and matter has transitioned from a theoretical concept to experimental reality in the past decade. This regime of quantum light-matter interaction goes beyond weak and strong coupling, where the coupling strength becomes comparable to the transition frequencies in the system. Achieving weak and strong coupling has enabled greater control of quantum systems and applications such as lasers, quantum sensing, and quantum information processing. This review discusses the theory of quantum systems with ultrastrong coupling, including entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to important physical models. It also covers various experimental setups, such as superconducting circuits, organic molecules, semiconductor polaritons, and optomechanics, that have achieved ultrastrong coupling. The review discusses the potential applications of these achievements in physics and chemistry. The interaction between light and matter is typically described by the fine structure constant α ≈ 1/137. Purcell's discovery that the interaction of an emitter with light can be enhanced or suppressed by engineering its electromagnetic environment led to the development of cavity quantum electrodynamics (CQED). The fundamental importance of controlling the strength of light-matter coupling g led to the development of resonators with higher quality factors. In 1983, Haroche and colleagues achieved a coupling strength exceeding the system losses, entering the strong-coupling (SC) regime. In this regime, vacuum Rabi oscillations occur, while in the weak-coupling (WC) regime, energy is lost before exchange between light and matter. The SC regime was later achieved with single atoms and optical cavities. In 1992, the SC regime was demonstrated using quasi-2D electronic excitations in a semiconductor optical microcavity. The resulting system eigenstates are called cavity-polaritons. Following these experiments, CQED has been adapted using artificial atoms such as quantum dots and superconducting qubits. In a CQED setup, the dimensionless parameter quantifying the interaction is the ratio between the coupling strength g and the bare energy of the excitations. This quantity, the normalized coupling η, is proportional to a positive power of α. The first observations of the SC regime had η values smaller than 10⁻⁶ for atoms and 10⁻³ for Wannier excitons. Perturbation theory is adequate for these experiments. The important difference with the WC regime is that, being the coupling larger than the spectral width of the excitations, degenerate perturbation theory needs to be applied. It took over two decades after the observation of SC for the CQED community to begin investigating the possibility to access a regime with larger η. Two main paths were identified to reach such a regime. The first is to couple many dipoles to the same cavity mode, leading to an enhanced coupling which scales with the square root of the number of dipoles. The second