New technology is needed by digital transmission networks for the increased bandwidth, performance and reliability requirements of existing and future telecommunication services, such as broadband ISDN. Increased flexibility, automation and control are also major issues being addressed. The combination of these factors has led to the development of synchronous digital heirarchy (SDH) networks to replace existing asynchronous networks during the 1990s. These issues are reviewed and further developments to bandwidth-transparent optical networks indicated for the twenty-first century.