Water samples were collected from 25 production and domestic wells in the Upper Hudson River Basin (north of the Federal Dam at Troy, N.Y.) from August through November 2007 to characterize the ground-water quality. The Upper Hudson River Basin covers 4,600 square miles in upstate New York, Vermont, and Massachusetts; the study area encompasses the 4,000 square miles that lie within New York. The basin is underlain by crystalline and sedimentary bedrock, including gneiss, shale, and slate; some sandstone and carbonate rocks are present locally. The bedrock in some areas is overlain by surficial deposits of saturated sand and gravel. Of the 25 wells sampled, 13 were finished in sand and gravel deposits, and 12 were finished in bedrock. The samples were collected and processed by standard U.S. Geological Survey procedures and were analyzed for 225 physical properties and constituents, including major ions, nutrients, trace elements, radon-222, pesticides, volatile organic compounds (VOCs), and indicator bacteria. Water quality in the study area is generally good, but concentrations of some constituents exceeded current or proposed Federal or New York State drinking-water standards; these were: color (1 sample), pH (2 samples), sodium (5 samples), nitrate plus nitrite (2 samples), aluminum (3 samples), iron (1 sample), manganese (7 samples), radon-222 (11 samples), and bacteria (1 sample). Dissolved-oxygen concentrations in samples from wells finished in sand and gravel [median 5.4 milligrams per liter (mg/L)] were greater than those from wells finished in bedrock (median 0.4 mg/L). The pH of all samples was typically neutral or slightly basic (median 7.6); the median water temperature was 9.7 deg C. The ions with the highest concentrations were bicarbonate (median 123 mg/L) and calcium (median 33.9 mg/L). Ground water in the basin is generally soft to moderately hard (less than or equal to 120 mg/L as CaCO3) (median hardness 110 mg/L as CaCO3). Concentrations of nitrate plus nitrite in samples from sand and gravel wells (median concentration 0.47 mg/L as nitrogen) were generally higher than those in samples from bedrock wells (median estimated 0.05 mg/L as nitrogen), and concentrations in two samples exceeded established drinking-water standards for nitrate (10 mg/L as nitrogen). The trace elements with the highest concentrations were strontium [median 217 micrograms per liter (ug/L)] and iron (median 39 ug/L). The highest radon-222 activities were in samples from bedrock wells [maximum 2,930 picocuries per liter (pCi/L)] and 44 percent of all samples exceeded a proposed U.S. Environmental Protection Agency (USEPA) drinking-water standard of 300 pCi/L. Ten pesticides and pesticide degradates were detected among 11 samples at concentrations of 1.47 ug/L or less; most were herbicides or their degradates. Six VOCs were detected among 10 samples at concentrations of 4.2 ug/L or less; these included three trihalomethanes and methyl tert-butyl ether, tetrachloroethene, and toluene. Most detections were in samples from sand and gravel wells and none exceeded drinking-water standards. Total coliform bacteria were detected in only one sample, and fecal coliform bacteria, including Escherichia coli, were not detected in any sample.