Abstract: Wireless sensor network are used in many fields now days like military application and ecolological area like flood detection and various other fields like health related. So the security of wsn is important issue. There are some constraints in wsn such as low battery and less memory which can lead to severe attacks in wsn. There are many attacks in wsn one of them is sybill attack. Sybill attack is an attack in which nodes illegally take multiple identities. In this paper we will discuss the taxonomy, classification of sybill attack and there defence mechanism.

Keywords: Sybil Attack, defence, Sensor Networks, Security, peer to peer system.