Daily data from the U.S. National Weather Service Cooperative Station Network are archived in digital form at the National Climatic Data Center for periods of record that generally begin in 1948. Under the Climate Database Modernization Program, all daily data that was previously available only on copies of manuscript forms has been digitized. If available on these manuscript forms, the values of were digitized for periods of record generally starting in the late 1800s and early 1900s and ending with the start of the currently archived digital data. Daily values of the following variables were keyed: maximum, minimum, mean temperature and temperature range; temperatures at the time of observation, 7:00 a.m., 2:00 p.m. and 9:00 p.m.; total precipitation, snowfall and depth of snow on the ground; prevailing wind direction and total wind movement; evaporation; sky condition; and the occurrence of weather and obstructions to vision. Precipitation amounts and temperature constitute the majority of the keyed data. Important quality issues that are addressed include assignment of proper identifiers to the data, changing observing and validation practices over time, incomplete documentation, and data inconsistencies. A quality assurance process was designed to eliminate obvious outliers and systematic internal inconsistencies, and suspect data are identified with a data-flagging scheme. The digitized daily data now have lengths of records that are decades longer than previously available. The longer periods of record can be used to solve important problems in areas of water management, climate variability, climate monitoring, and risk assessment.