There are many kinds of systems developed for anonymous communication on the internet. We survey a number of systems and evaluate their security. Among these systems we compare functionalities like Onion Routing, anonymous VPN services, probabilistic anonymity, and deterministic anonymity. Other types of anonymous communication such as messaging, peer-to-peer communication, web use, emailing, and use of other Internet applications are also presented. We follow up by presenting different types of attacks with the purpose of identifying anonymously communicating users. These attacks fall into the following categories: internal/external attacks, passive/active attacks, and static/adaptive attacks. We describe the following attacks as well as known protections against these attacks: predecessor attacks, intersection attacks, timing attacks, and Sybil attacks. Lastly we discuss design choices, operation, and security of the current TOR network – The 2G Onion Router. Access control methods to restrict malicious use of TOR are also proposed. In conclusions the significance of anonymous communication is outlined.