Abstract: Quantum sensing is one of the core directions of current quantum technology, and has the longest history, the highest maturity, and the widest applications. This article reviews the basic theory and methods of quantum sensing. First, the definition and basic concepts of quantum sensing are summarized, including the origin of the quantum nature of quantum sensing and the extension and classification of quantum sensing from the perspective of practical application. The basic and general protocol of quantum sensing is then described in detail, as well as the core technical parameters for depicting the performance of quantum sensing. Finally, the technical methods and physical principles to improve the performance of quantum sensing are summarized.