Systems Network Architecture (SNA) has evolved from an architecture that supported implementation of tree networks rooted in a System/370 to an architecture that supports multiple-system networks with capabilities such as alternate paths and parallel links. This paper describes the major SNA enhancements that have been implemented for multiple-system networks. As network configurations have become more complex, the problems associated with network growth, change, failures, recovery, and flow control have required solutions that permit continuous network operation. The SNA enhancements that address these problems are also discussed.