ATM technology is expected to gain widespread use both in wide-area networks and in high-speed local area networks. The performance of the TCP/IP protocol suite over such networks is of great importance, as it is widely used in the Internet and in private internetworks. However, numerous studies have shown that the effective throughput of conventional TCP implementations suffers over plain ATM networks. This thesis presents a detailed study of the interactions between TCP's congestion control mechanisms and ATM networks. Based on the results of this study, we propose an enhanced version of TCP Vegas (called new-Vegas) and show that it achieves an increase in throughput of 40-70% over TCP Lite on plain ATM networks and up to 20% on EPD-enhanced ATM networks and packet networks. Moreover, our results indicate that the enhanced TCP Vegas is largely insensitive to EPD/PPD. In fact, even on plain ATM networks, it performs within 7% of its best throughput.