My dissertation is a randomized trial and part of a large NSF-supported study in our lab on embedding assessments in the FAST (Foundational Approaches in Science Teaching) middle school science curriculum to promote students' science learning about why things sink and float. It is designed to answer three specific research questions: (1) Can formative assessment improve students' motivational beliefs? (2) Can formative assessment improve students' achievement in science (including conceptual change)? (3) Is students' science achievement (including conceptual change) correlated with their motivational beliefs? The dissertation examines the influence of formative assessment the embedded assessments and the corresponding feedback to students' regarding their performance with those embedded assessments on students' science achievement and motivation. In particular, science achievement refers to students' conceptual change in explaining buoyancy phenomena and their performance in solving related science problems; motivation focuses on motivational beliefs conceived to contribute to conceptual change and be fostered by formative assessment.