To fulfill global requirements for network security and privacy, anonymous communication systems have been extensively investigated and deployed over the world to provide anonymous communication services for users. Nonetheless, diverse de-anonymizing techniques have been proposed to compromise anonymity and impose a severe threat to anonymous communication systems. In this article, we classify the existing de-anonymizing techniques and provide an overview of these techniques. In addition, corresponding countermeasures are studied to mitigate the risks posed by these de-anonymizing techniques.