Superconductivity manifests macroscopic quantum phenomena. A superconducting circuit composed of Josephson junctions, Cooper pair boxes, and rf-/dc-SQUID, properly miniaturized, becomes quantized and demonstrates Rabi oscillations and entanglement. In this chapter, we begin with an introduction of the history and elementary theory of superconductivity. Then we describe the building blocks of superconducting classical circuits and derive their canonical quantizations. The set up of qubits and superconducting quantum logic gates are then examined. Finally, ways to make measurements are discussed. We should remark that SQUID are widely regarded as the most scalable quantum computing device.