NARCCAP is a new international program that will serve the climate scenario needs of both the United States and Canada. We are systematically investigating the uncertainties in regional scale projections of future climate and producing high resolution climate change scenarios using multiple regional climate models (RCMs) and multiple global model responses to future emissions scenarios, by nesting the RCMs within multiple atmosphere ocean general circulation models (AOGCMs) forced with the A2 and A1B SRES scenarios, over a domain covering the conterminous US and most of Canada. The plan also includes a validation aspect through nesting the participating RCMs within reanalyses. The basic spatial resolution of the RCMs is 50 km. This program will include RCMs that participated in the European PRUDENCE program (HadRM3 and RegCM), the Canadian regional climate model (CRCM) as well as the NCEP regional spectral model (RSM) and MM5. Candidate AOGCMs include the Hadley Centre HadCM3, NCAR CCSM, the Canadian CGCM3 and the GFDL model. The resulting climate model runs will form the basis for multiple high resolution climate scenarios that can be used in climate change impacts assessments in the US and Canada. High-resolution global time slice experiments based on the GFDL atmospheric model and the NCAR atmospheric model (CAM3) will also be produced and will be compared with runs of the regional models. There also will be opportunities for double nesting over key regions through which additional modelers in the regional modeling community will be able to participate in NARCCAP. Additional key science issues will be investigated such as the importance of compatible physics in the nested and nesting models. Measures of uncertainty across the multiple runs will be developed by geophysical statisticians.