Basic data on the surface water, ground water, and water quality of Cortland County, New York, are presented with a short explanatory text. Seepage investigations showed that during periods of base flow, the surface-water regime cannot be predicted on the basis of flow-duration figures alone. The investigations also indicate that the streambeds are permeable for extended reaches throughout the areas studied. Inconsistent data on gain or loss for a given reach are assumed to be indicative of the changes in rate of ground-water recharge or discharge. Analysis of surface-water quality since 1972 showed high nitrate (as NO3) concentrations (4 to 18 mg/liter) in the Cortland area and in Factory Brook. The nitrate source is probably from human activities. Although ground water in the Gridley Creek basin is moderately hard (34 to 140 mg/liter CaCO3), it generally meets standards of the New York State Department of Health. (Woodard-USGS)