Abstract: Quorum system is a novel set system with redundant topology Based on the redundancy design, a quorum duplicates its data to other intersected quorums by intersected points, which increases the data redundancy of Quorum systems Whenever any malfunctions or errors happen on any points of quorums, Quorum systems can run availably by executing voting protocol, whose valid data are voted from remaining valid points on one quorum, and mutual exclusion protocol, whose valid data are obtaioed from all points of one remaining valid quorum Various fault tolerant methods for different quorum systems are analyzed and their performances are compared, and some existing problems are explored Finally proposed are some key issues and possible research trend of the development of quorum systems in the future