Systems and methods are provided for intrusion detection. The systems and methods may include receiving transaction information related to one or more current transactions between a client entity and a resource server, accessing a database storing a plurality of transaction groups, analyzing the received transaction information with respect to information related to at least one of the plurality of transaction groups, and based on said analyzing, determining a possibility of an occurrence of an intrusion act at the resource server. The transaction groups may be formed based on a plurality of past transactions between a plurality of client entities and the resource server. Identity information of a user associated with the one or more current transactions may also be received along with the transaction information. The user may be associated with at least one of the plurality of transaction groups.