The Transmission Control Protocol (TCP) and Internet Protocol (IP), or, together, TCP/IP, form the complex but essential language that allows hosts on the Internet to communicate. TCP/IP is the industry standard for interconnecting hosts and has been so widely adopted that it is essentially the default protocol of the Internet. It provides a common interface for different types of applications that is independent of the physical network. It underpins many higher layer applications, such as the World Wide Web, email, HTTP, File Transfer Protocol, Telnet, Simple Mail Transfer Protocol, and streaming media. For these reasons, it is sometimes seen as the “engine behind the Internet.” The entry examines the technical dimensions of TCP/IP and how the protocol developed over time.