The National Science Festivals Project is designed to create a sustainable national network of science festivals that engages all facets of the general public in science learning. Science Festivals, clearly distinct from "science fairs", are community-wide activities engaging professional scientists and informal and K-12 educators targeting underrepresented segments of local communities historically underserved by formal or informal STEM educational activities. The initiative builds on previous work in other parts of the world (e.g. Europe, Australasia) and on recent efforts in the U.S. to create science festivals. The target audiences are families, children and youth ages 5-18, adults, professional scientists and educators in K-12 and informal science institutions, and underserved and underrepresented communities. Project partners include the MIT Museum in Cambridge, UC San Diego, UC San Francisco, and the Franklin Institute in Philadelphia. The deliverables include annual science festivals in these four cities supported by year-round related activities for K-12 and informal audiences, a partnership network, a web portal, and two national conferences. Ten science festivals will be convened in total over the 3 years of the project, each reaching 15,000 to 60,000 participants per year. STEM content includes earth and space science, oceanography, biological/biomedical science, bioinformatics, and computer, behavioral, aeronautical, nanotechnology, environmental, and nuclear science. An independent evaluator will systematically assess audience participation and perceptions, level/types of science interest stimulated in target groups, growth of partnering support at individual sites, and increasing interactions between ISE and formal K-12 education. A variety of qualitative and quantitative assessments will be designed and utilized. The project has the potential to transform public communication and understanding of science and increase the numbers of youth interested in pursuing science.