The North American Regional Climate Change Assessment Program (NARCCAP) is a collection of regional climate model simulations downscaling global simulations from CMIP3 to 50-km resolution over North America. The collection was generated in 2007-2012 with the goal of investigating uncertainties in regional scale projections of future climate and generating climate change scenarios for use in impacts research. NARCCAP comprises a set 12 simulations from 6 RCMs downscaling 4 GCMs using a fractional factorial design, plus 1 simulation from each RCM downscaling the NCEP reanalysis, and 2 global atmosphere-only timeslice experiments. Historical data spans 1971-2000, and future data 2041-2070 using the SRES A2 emissions scenario. It includes more than 3 dozen 2D variables and a half-dozen 3D variables at 3-hourly frequencies, plus a handful of static and daily variables. All data is at 50-km spatial resolution over a domain that covers most of North America and is stored in CF-compliant netCDF files. More detailed documentation of the dataset is available at: https://narccap.ucar.edu