Exploratory data from more than 1000 analyses of the distribution of deuterium in waters of the North American continent and the surface oceans contiguous to the continent are presented. The elementary theory of the processes that appear to explain the changes in the deuterium content of natural waters is developed. Quantitative expressions of the deuterium fractionation that can be expected to occur are presented for all phases of the hydrologic cycle from the evaporation of water from the oceans, its precipitation as rain and snow, and its travel back to the sea. Processes such as the freezing of water under equilibrium and nonequilibrium conditions, the evaporation of water from closed lakes and from lakes with an outlet, the formation of fog, frost, and dew are also discussed. The regional characteristics of the surface waters of North America are described and interpreted as reflecting the history of the water in the course of the hydrologic cycle.