Summary form only given. The initial application of WDM technology in long distance transport networks has been a large success. It solved a severe capacity problem quickly, and at a cost much lower than possible with the alternatives available. This tutorial will examine WDM networking systems, both present and future, from the point of view of a telecommunications network operator. It will first examine today's WDM network applications, including the service drivers, the economic factors, and the way it is integrated with other elements of the transport solution. It then moves on to examine how the role of WDM systems will change in the future. What services will expand WDM technology deployment beyond long-distance network applications? What problems will it solve? How will WDM compare with other alternatives? Finally, how will WDM transport solutions fit in with the other networking layers to create an overall transport solution? It will attempt to predict the evolution of WDM networking technology, moving from today's primarily transmission technology, to a fully functional networking layer. How will this be accomplished in an evolutionary way, continually solving real network problems, and doing so in an economical way?.