Since their first serious introduction in the early 1 960s, systems analysis methods have held out a promise of providing information that can help improve decision making. This paper surveys a number of actual applications of optimization and simulation modelling that illustrate the variety of problems studied, the variety of approaches used to study them, and the variety of outcomes or results of such studies. Some studies have been very successful, some have not. Given the varying but generally beneficial results of such studies, it seems clear that research on, together with application of, systems analysis techniques to water resources and environmental problems should continue. The paper concludes with some suggestions for facilitating the development and effective application of this methodology.