Prior research suggests that students' understanding of scientific concepts is pre-determined by their reasoning ability. Other efforts suggest that American students' scientific literacy is in decline. One difficulty Bybee (2009) acknowledges is that there are two divergent philosophical models of scientific literacy. The first describes the content knowledge and conceptual understanding that is desirable for future scientists. The second is an application of science to "real life" that is critical for every American citizen. In either case, we propose that the essential subordinate skill required for scientific literacy is scientific thinking ability as defined by Piaget (1964) and Lawson (2002). This article outlines the relation between students' scientific thinking ability and both their conceptual understanding and literacy. It also provides national norms for students in grades 10-12 using a measure of scientific thinking skills, the Classroom Test of Scientific Reasoning (CTSR). These norms can be used by educators when making curricular and policy decisions regarding student achievement. (Contains 1 table and 5 figures.)