Structured Peer to Peer (P2P) networks provide efficient mechanisms for resource placement and lookup. However, these systems deal with irregular and frequent arrival/departure of nodes. Thus, these systems do not offer any guarantees about data availability. A well-known technique for improving resources availability and providing load balancing is replication. Many replication methods are proposed for structured P2P networks, with a specific main goal to achieve. This paper reviews and compares various existing replication techniques for structured P2P networks, which we classify based on their main objectives.