In this article some fundamental issues in hydrologic modeling are discussed, and some emerging methods toward their solutions are presented. INTRODUCTION Many of the hydrologic modeling tools date back to 1960's, such as Sugawara's tank model, and, later, the Stanford watershed model, which formed the basis for the conceptual watershed hydrology models that have been used by government agencies and practitioners all around the world. These models are still being in current use as the main tools for hydrologic water balance studies, environmental modeling studies, flood forecasting, seasonal flow forecasting, etc. Meanwhile, for the risk-based analysis and design of hydraulic structures the standard methods have been the flood frequency analysis and the probable maximum flood estimation. The flood frequency analysis procedure was standardized in the USA in mid-1970s by the development of the Bulletin 17 by the US Water Resources Council (1976). This standardized flood frequency analysis method was revised in 1982 by the US Department of Interior Interagency Advisory Committee on Water Data (1982), and has been used for more than 30 years until present by all US Agencies and water resources engineering practitioners in the US and elsewhere around the world. As for the design of very large hydraulic structures and nuclear power plants, the probable maximum precipitation procedure was developed by the US Weather Bureau in mid-1950s (US Weather Bureau, 1956). While it has gone through some minor revisions through a series of Hydrometerological Reports, this approach to the estimation of probable maximum precipitation was adopted by World Meteorological Organization (WMO, 1986), and have been practiced around the world. The probable maximum flood has been estimated by rainfall-runoff models that used various-duration probable maximum precipitation estimates as their input. With the emergence of climate change as a major issue for the earth system, many standard methods in hydrologic modeling that are based on the fundamental assumption of statistical equilibrium of the hydro-climate of a study region, are now in question for their scientific basis. Furthermore, sparseness or lack of atmospheric and/or hydrologic data at many regions of the world have prevented rigorous hydrologic modeling efforts for the assessment of water balances and