The article provides an overview of hybrid quantum circuits, which combine two or more physical systems to leverage the strengths of each system for new phenomena and potential quantum technologies. It discusses the integration of atomic systems, spins, and solid-state devices (including superconducting and nanomechanical systems) into hybrid circuits. The article highlights the advantages and challenges of each system, such as long coherence times for atoms and spins, and scalability for superconducting circuits. It also covers the theoretical principles and experimental progress in coupling these systems to cavities and resonators, including cavity quantum electrodynamics (QED) and the use of superconducting qubits. The article emphasizes the importance of hybrid circuits in quantum information processing, quantum computing, and quantum simulation, and discusses the potential for future applications in quantum detectors, simulators, and computers.The article provides an overview of hybrid quantum circuits, which combine two or more physical systems to leverage the strengths of each system for new phenomena and potential quantum technologies. It discusses the integration of atomic systems, spins, and solid-state devices (including superconducting and nanomechanical systems) into hybrid circuits. The article highlights the advantages and challenges of each system, such as long coherence times for atoms and spins, and scalability for superconducting circuits. It also covers the theoretical principles and experimental progress in coupling these systems to cavities and resonators, including cavity quantum electrodynamics (QED) and the use of superconducting qubits. The article emphasizes the importance of hybrid circuits in quantum information processing, quantum computing, and quantum simulation, and discusses the potential for future applications in quantum detectors, simulators, and computers.