Henry Darcy and Jules Dupuit were born 1 year apart, were classmates during their undergraduate and graduate education in civil engineering, and were colleagues in the French corps of civil engineers, with overlapping appointments as inspector general in the early 1850s. At that time Darcy turned over, to Dupuit, his position as Director of Water and Bridges in Paris and the research on pipe flow he had begun there in 1849. In these pipe flow experiments, Darcy discovered what he referred to as a “law” of fluid mechanics, which is that above a certain velocity threshold, the head loss is proportional to velocity squared, and below that threshold, the head loss is linearly proportional to velocity. During the remainder of their careers, Darcy and Dupuit applied this law with their collective, extensive, prior knowledge of fluid mechanics, geology, aquifers, wells, and springs to quantitative studies of fluid flow in the subsurface (and also in pipes, aqueducts, rivers, and sand filters). Two monographs by Darcy (1856) and Dupuit (1857) are mutually cited retrospectives on much of this research, submitted at nearly the same time, to the same Corps des Ponts et Chaussées publisher, near the end of their careers. Between these two monographs, many of the fundamentals of quantitative hydrogeology were established, including the equation for groundwater motion, average linear velocity, average travel time, effective hydraulic conductivity for layered heterogeneity, conservation of mass in confined and unconfined flow, the nature of the regional pieziometric surface, porous flow versus flow through discrete fractures and karst conduits, the equation for a cone of depression around flowing wells, superposition of the effects of multiple wells, and capture zone geometries of wells within a regional flow field.