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There is an increasing command for anonymity in network. Crowds[1] is an popular anonymity system proposed by Michael K. Reiter and Aviel D. Rubin which main idea behind it is hiding users’ identities by routing their packets randomly within a group of similar users. Crowds can provides sender anonymity and also has an advantage that the computation load of relay nodes is very small but it does not protect the identity of the receiver. This paper presents an overview on Crowds and introduces the development of Crowds from the following aspects: anonymity analysis, application especially the application in P2P and the performance. We summarize the work about anonymity analysis based on Crowds and divide them into receiver analysis, probabilistic behavior, attack and anonymity proof. Crowds is high efficient but it can’t resist the global attack, therefore, to meet the needs of the different levels of anonymity, there is a vast body of anonymity literature concerned with anonymity analysis, for example, Crowds can’t provide receiver anonymity but receiver anonymity is important in many cases, so many work extending the Crowds system from sender anonymity only to sender and receiver anonymity. In addition to the analysis, we also summarize the applications of Crowds. Undoubtedly, it is widely used in P2P. We also give an overlook of the performance of Crowds.