The purpose of this study was to explore (a) the individual belief profiles that naturally arise among middle and high school science students (n = 1225); (b) the relationships between these profiles to science achievement and other prominent motivation variables; and (c) the demographic and developmental differences among the belief profiles. Results revealed that a four-class solution fit the data the best. These profiles were differentially related to achievement goal orientations, science self-efficacy, and science achievement. Differences in profiles also arose as a function of minority status, grade level, and gender. Findings support and refine Schommer-Aikins's (2004) Embedded Systemic Model of epistemic beliefs. Results are discussed in relation to theory and implications for science instruction. (Contains 3 figures and 4 tables.)