As the name suggests, voice over Internet Protocol (VoIP) refers to the technology that allows transmission of voice over IP networks. Using VoIP, we can make telephone calls over the Internet or any IP-based corporate networks. With VoIP in place, the Internet does not remain a data-only network; it becomes a telephone network as well. So, what is the motivation for VoIP? There are several appealing reasons why we want this technology. The most compelling reason behind the emergence of this technology is the enormous cost savings through integration of both voice and data, two large revenue-making media for telecommunication companies, over the same communication infrastructure. The other major motivation for VoIP is the widespread adoption of IP platform. Today IP is a well-entrenched technology which is deployed in servers, routers, desktop PCs, laptops, and in many hand-held mobile devices, such as palmtops and PDAs. If voice signals could be transported over IP, then voice could be delivered to all these devices, making these devices much more useful than they already are. This chapter discusses several aspects of VoIP: (1) Advanced applications powered by VoIP, (2) the technical challenges and solutions for making VoIP a reality, (3) various VoIP configurations, and (4) the protocols and standards for VoIP. We conclude the chapter by discussing the future of VoIP.