This paper provides an introduction to, and an overview of, the Special Issue on the application of systems approach to the management of complex water systems. The main motivation in proposing this Special Issue was that today, more than ever, we need a systems approach to assist in dealing with the difficulties introduced by the increase in the complexity of water resource problems, consideration of environmental impacts, and the introduction of the principles of sustainability. This issue offers an opportunity to review applications of the systems approach to water resource management and draw lessons from worldwide experience relevant to future water problems. The Special Issue includes 15 contributions that offer an interesting view into contemporary problems, approaches, and issues related to management of complex water resources systems. It will be presumptuous to say that these 15 contributions characterize the success or failure of the systems approach to support water resources decision-making. However, these contributions offer some interesting lessons from the current experience and trace possible future work directions.