Voice over IP (VoIP) enables a voice-enabled router to carry voice traffic, such as telephone calls and faxes, over an IP network. This chapter introduces the fundamentals of VoIP, architecture types, and available voice-signaling protocols. Numbering plans, dial plans, and VoIP access to 911 emergency services are explained. The role of gateways and their use in integrating VoIP with traditional voice technologies is described. Traffic engineering and bandwidth calculations are discussed. Finally, the impact of security threats and the components required for a secure voice network are also explained.