Natural variations of oxygen 18 and deuterium abundances in groundwaters, as compared to the local precipitation, are discussed as a hydrologic tool. It is shown that the residual scatter in time and space of the isotope content of groundwater sources is a measure of the homogeneity of these systems rather than of their age or size and that the concept of a regional slope of evaporation lines on the δ_D–δ_{18 O} diagram has only limited validity. Isotope criteria are suggested for distinguishing rainfall from past periods of different climate from present rainfall. It is concluded that as a result of the unsteady nature of the mechanisms affecting the isotope composition of groundwaters, their most reliable use, under the limitations of our present quantitative knowledge on the causes of this scatter, is as classifying parameters in regional investigations.