The past four years have seen various statistical and optimization‐simulation modeling techniques of increased quality and usefulness developed and applied to water resources system design and operating problems. Older modeling approaches continue to be improved and newer methods continue to be developed for the comprehensive analysis of larger complex problems. What distinguishes the last four years from the past is the major increase in the number of successful and useful applications of these mathematical modeling techniques. If success can be measured by the impact models have on our understanding and management of hydrologie or water resource systems, then any review of the recent literature (especially of the proceedings of various symposia and conferences on this subject) will demonstrate that these methods of analyses have matured and are being successfully used throughout the world.