Computer telephony integration (CTI) enhances the capability and flexibility of the switches or the network end equipment by computer technology. This article introduces the components of CTI such as switch-to-host interfaces, application programming interfaces, and resource architectures. Then we describe two CTI configurations: first-party call control and third-party call control. CTI-based telecommunication services such as unified messaging and call center are described. We also illustrate the Integrated Office Telephony System (IOTS), a CTI system developed at Computer Communication Research Laboratories, which integrates the switch and CTI server on a personal computer platform.