Quantum cryptography,which is based on fundamental physical principles,has been proved to be an effective technique for secure key distribution.In this paper,the authors introduce the fundamental theory of quantum cryptography,then discussed the three main schemes for quantum cryptography and their key distribution policies respectively.Recent improvements in theory and experiment are discussed,and the existng problems and developing directions are also pointed out.For its application potential,it's suggested to construct experimental system of quantum cryptography in China.