Large numbers of U.S. students lack proficiency in science, and students from different ethnic and socioeconomic groups show disparities in science achievement. Science knowledge and skills are important for both academic and workplace success, and a variety of interventions have been developed to improve student achievement in science. The "Full Option Science System™" (FOSS) is a science curriculum for students in kindergarten to grade 8 with content in physical science, earth science, and life science. The curriculum consists of a series of 8- to 9-week modules in kindergarten to grade 5, and 9- or 18-week courses in grades 6 to 8. This What Works Clearinghouse (WWC) report, part of the WWC's Primary Science topic area, examines research on the effects of FOSS on science achievement for students in kindergarten through grade 8. No studies of FOSS that fall within the scope of the Primary Science review protocol meet WWC standards. Because no studies meet WWC standards, the WWC is unable to draw any conclusions at this time about the effectiveness of FOSS on science achievement.