IDPS technologies may be classi±ed according to diferent parameters namely: the methodologies they employ to detect intrusions: signature-based detection, anomaly-based detection and stateful protocol analysis. The functionalities they provide, which ultimately di f erentiate passive systems (IDSs) from re-active systems (IPSs). The type of events they monitor, which are closely related to the type of systems they guard: a wired network, a wireless network or a single host. In addition to these, a fourth type of IDPS may be identi±ed, which is known as Network Behavior Analysis (NBA) IDPS.