The article reviews the theory and experimental progress in ultrastrong coupling between light and matter. Ultrastrong coupling, a regime where the coupling strength is comparable to the transition frequencies of the system, has emerged as a new regime of quantum light-matter interaction, surpassing both weak and strong coupling. The review covers the theoretical framework, including entangled ground states with virtual excitations and connections to nonlinear optics, as well as experimental setups such as superconducting circuits, organic molecules, semiconductor polaritons, and optomechanics. It discusses the potential applications in physics and chemistry, highlighting the ability to control and manipulate quantum systems at unprecedented levels. The article also explores the unique properties of ultrastrongly coupled systems, such as the presence of virtual excitations in the ground state and the impact of the diamagnetic term on the system's behavior.The article reviews the theory and experimental progress in ultrastrong coupling between light and matter. Ultrastrong coupling, a regime where the coupling strength is comparable to the transition frequencies of the system, has emerged as a new regime of quantum light-matter interaction, surpassing both weak and strong coupling. The review covers the theoretical framework, including entangled ground states with virtual excitations and connections to nonlinear optics, as well as experimental setups such as superconducting circuits, organic molecules, semiconductor polaritons, and optomechanics. It discusses the potential applications in physics and chemistry, highlighting the ability to control and manipulate quantum systems at unprecedented levels. The article also explores the unique properties of ultrastrongly coupled systems, such as the presence of virtual excitations in the ground state and the impact of the diamagnetic term on the system's behavior.