The idea of a globalhydrologic cycledates back at least 3000 years when early Greek philosophers including Thales, Anaxagoras, Herodotus, Hippocrates, Plato, and Aristotle conceptualized the basic ideas governing the process. Many initial ideas established by the Greeks about the hydrologic cycle were reasonable. However, many of the initial mechanisms concerning the routes by which water returned from the sea and entered rivers were devoid of as much logic. Despite the apparent gaps in hydrologic mechanisms, the Romans developed aqueduct systems reflecting an extensive practical understanding of hydrology and hydraulics, and did so utilizing the basic hydrologic ideas established and passed along by the Greeks (Dingman 1994). During the Renaissance,Leonardo da Vinci(1500) in France proclaimed on the basis of field observations that the waters in rivers come fromprecipitation. It was during that time that any unrealistic mechanisms proclaimed by the Greek philosophers concerning the hydrologic cycle were either refuted or modified. In the seventeenth century, the modern scientific approach to studying the hydrologic cycle was initiated by the Frenchmen Pierre Perault and Edme Marriotte. By the 1670s and 1680s, they had published data and calculations that supported the contention that precipitation was the precursor to stream flow. By 1700, Edmun Halley, an English scientist added to the work of Perault and Marriotte by estimating the quantity of water involved in the hydrologic cycle of the Mediterranean Sea and surrounding lands.Substantial progress was made during the eighteenth century in applications of mathematics, fluid mechanics, and hydraulics by scientists like Pitot, Bernoulli, Euler, Chezy, and other European professionals. The term hydrology arrived in its current meaning around 1750, and by 1800 the work of English physicist and chemistJohn Daltonsolidified the current understanding of the global hydrologic cycle.Until the 1800s, the physical processes governinggroundwaterflow had confounded scientists and created barriers to understanding thehydrologic cycle. These barriers were eliminated in 1856, when the French engineerHenry Darcyintroduced his law describing flow through porous media. Other advances in the hydrological sciences were made throughout the 1800s. Poiseuille, DuPuit, DuBoys, Stokes, Manning, Reynolds, and others made substantial contributions to fluid mechanics, hydraulics, and sediment transport during this period. Also during the 1800s, literary publications began to surface, with increasing frequency in the last half of the century. Many works examined relationships betweenprecipitationandstreamflowout of necessity for engineers designing bridges and other structures. It was during this time that the close association between hydrology and civil engineering was established. Daniel Mead published the first English-language text in hydrology in 1904 and Adolf Meyer followed with his text in 1919. Both texts were written for civil engineers. The association of hydrology and civil engineering established during this time has been argued as having both enhanced as well as possibly inhibited the development of hydrology as a science.The first half of the twentieth century saw great advancements in the hydrological sciences starting with the addition of the Section of Scientific Hydrology in the International Union of Geodesy and Geophysics in 1922. This was followed by the addition of the Hydrology Section of the American Geophysical Union in 1930. These were the first formal recognitions of the scientific status of hydrology. Many individuals contributed substantially in their areas of hydrologic expertise during the early and middle decades of the 20th century, including: A. Hazen, E. J. Gumbel, H. E. Hurst, and W. B. Langbein with regard to statistical applications of hydrologic data; O. E. Meinzer, C. V. Theis, C. S. Slichter, and M. K. Hubbert who contributed to the development of theoretical and practical aspects of groundwater hydraulics; L. Prandtl, T. Von Karman, H. Rouse, V. T. Chow, G. K. Gilbert, and H. A. Einstein in sediment transport and stream hydraulics; R. E. Horton and L. B. Leopold who contributed greatly to runoff processes and quantitative geomorphology; W. Thornthwaite and H. E. Penman in furthering the understanding of hydroclimatalogical processes and modelingevapotranspiration; and A. Wolman and R. S. Garrels who contributed greatly to the understanding and modeling of water quality. It was not until the 1960s that detailed field studies attempting to understand the physical processes by which water enters streams began to emerge.With the emergence of the twenty-first century, many new breakthroughs in the hydrological sciences are eminent. In the forthcoming years, breakthroughs will describe the relationships between hydrological regimes to current and futureclimate change, and the effects of hydrologic processes onlandform development. New findings will also include modeling of regional evapotranspiration rates and geomorphologic water transport.