Superconducting circuits based on Josephson junctions can behave like artificial atoms, exhibiting macroscopic quantum coherence. These circuits have enabled the implementation of atomic-physics and quantum-optics experiments on a chip. This review discusses phenomena analogous to those in natural atoms and highlights unique features of superconducting circuits. Superconducting circuits can be used as qubits for quantum computing and exhibit striking quantum behaviors despite limited decoherence times. These circuits can have discrete energy levels and behave like superconducting artificial atoms, with a deep analogy to natural atoms. Differences include energy scales and environmental coupling. Superconducting circuits can be designed with specific characteristics and fabricated on a chip, offering tunability advantages over natural atoms. These circuits can test quantum mechanics at a macroscopic scale and demonstrate atomic physics and quantum optics on a chip. Artificial atoms can have exotic properties not found in natural atoms. Superconducting circuits can be used for cavity quantum electrodynamics, where a quantized electromagnetic field exchanges energy with a two-level system. This process involves Rabi oscillations and has been demonstrated in superconducting circuits. These circuits can achieve strong coupling with cavity modes, enabling the observation of the Lamb shift. Superconducting circuits can also be used for frequency conversion, with a Δ-type artificial atom allowing upconversion and downconversion of photon frequencies. Electromagnetically induced transparency (EIT) can be achieved in superconducting circuits, with applications in slowing and stopping light. State population inversion and lasing can be achieved in superconducting circuits, with lasing observed in experiments using Cooper-pair boxes. Cooling techniques, such as Sisyphus cooling and inverse SPI, can be used to cool superconducting qubits. Photon generation and quantum communication can be achieved using superconducting circuits, with single-photon sources and quantum state transfers demonstrated. Quantum state tomography and process tomography can be used to measure and characterize quantum states and processes. Future prospects include testing quantum mechanics on a macroscopic scale and demonstrating novel quantum phenomena. The Bell, Leggett-Garg, and Kochen-Specker inequalities have been tested using superconducting circuits, showing quantum behavior. Superconducting circuits can also be used for interferometry and quantum nondemolition measurements, with applications in quantum optics and technology.Superconducting circuits based on Josephson junctions can behave like artificial atoms, exhibiting macroscopic quantum coherence. These circuits have enabled the implementation of atomic-physics and quantum-optics experiments on a chip. This review discusses phenomena analogous to those in natural atoms and highlights unique features of superconducting circuits. Superconducting circuits can be used as qubits for quantum computing and exhibit striking quantum behaviors despite limited decoherence times. These circuits can have discrete energy levels and behave like superconducting artificial atoms, with a deep analogy to natural atoms. Differences include energy scales and environmental coupling. Superconducting circuits can be designed with specific characteristics and fabricated on a chip, offering tunability advantages over natural atoms. These circuits can test quantum mechanics at a macroscopic scale and demonstrate atomic physics and quantum optics on a chip. Artificial atoms can have exotic properties not found in natural atoms. Superconducting circuits can be used for cavity quantum electrodynamics, where a quantized electromagnetic field exchanges energy with a two-level system. This process involves Rabi oscillations and has been demonstrated in superconducting circuits. These circuits can achieve strong coupling with cavity modes, enabling the observation of the Lamb shift. Superconducting circuits can also be used for frequency conversion, with a Δ-type artificial atom allowing upconversion and downconversion of photon frequencies. Electromagnetically induced transparency (EIT) can be achieved in superconducting circuits, with applications in slowing and stopping light. State population inversion and lasing can be achieved in superconducting circuits, with lasing observed in experiments using Cooper-pair boxes. Cooling techniques, such as Sisyphus cooling and inverse SPI, can be used to cool superconducting qubits. Photon generation and quantum communication can be achieved using superconducting circuits, with single-photon sources and quantum state transfers demonstrated. Quantum state tomography and process tomography can be used to measure and characterize quantum states and processes. Future prospects include testing quantum mechanics on a macroscopic scale and demonstrating novel quantum phenomena. The Bell, Leggett-Garg, and Kochen-Specker inequalities have been tested using superconducting circuits, showing quantum behavior. Superconducting circuits can also be used for interferometry and quantum nondemolition measurements, with applications in quantum optics and technology.