 are now facing security problems that have not been previously encountered. Contrariwise to PSTN, which is based on a closed network architecture and thus it neither experiences a big variety of attacks nor a high attack frequency [2], VoIP utilizes an open network architecture and thus VoIP providers and end users should deal not only with the Internet's native vulner- abilities and attacks but also with VoIP oriented ones. Several researchers [3-6] have identified various security flaws in VoIP signaling protocols, like Session C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Initiation Protocol (SIP) [7], H.323 [8] and IAX [9]. These security flaws can be exploited by attackers in order to achieve either unauthorized access or to cause Denial of Service (DoS). It is stressed that the majority of the published research work focuses on SIP, because SIP has been adopted by various stan- dardization organizations as the predominant protocol for both wireline and wireless in the Next Generation Networks (NGN) era. The rather straight- forward way to access the communication channel combined with SIP's text- based message structure, makes SIP an attractive target for attackers. Without doubt, in order to gain global acceptance, it is necessary for VoIP to offer at least similar levels of security, availability and reliability to those of PSTN. This chapter focuses on SIP security issues. Particularly, it presents and thoroughly analyzes existing vulnerabilities, threats and attacks against SIP. Moreover it identifies specific security requirements that the SIP-dedicated security mechanisms and the corresponding Intrusion Detection Systems (IDS) must fulfill in order to eliminate the respective security flaws. 18.2 SIP Threats and Vulnerabilities Prior to the design and development of any new service, critical or not, the identification of potential risks and vulnerabilities should pave the way towards the adoption of the appropriate security countermeasures; in accor- dance t