As one of the most important group-oriented signatures, ring signatures were initially formalized by Rivet et al. and allows an entity to conscript a group of ring members at will and sign a message in the name of the group without revealing a real identity. Featured with spontaneity and unconditional anonymity, ring signatures have found widespread applications in fair exchange, electronic commerce, wireless sensor networks and vehicular ad hoc networks. A survey of ring signatures is presented in this book to provide researchers and scholars with a better understanding of this primitive cryptography. To be specific, we survey state-of-the-art ring signature schemes insightfully according to the design philosophies of these schemes. The existing schemes have also been classified in view of various perspectives. Furthermore, the security model and efficiency of existing schemes have been compared. On the other hand, the extensions of ring signature schemes, such as proxy ring signature, threshold ring signature, ring signcryption, deniable ring signature, linkable ring signature and conditional anonymous ring signature, are also investigated. Finally, the challenges and future directions of ring signatures, such as the construction with constant-size, the efficiency and provably-secure construction in the standard model, and new application scenarios have also been given.