A central challenge in efficacy studies centers on the issue of "fidelity of implementation," that is, the extent to which participants use the curriculum specified by curriculum developers. In this study, we describe and discuss a "fidelity of implementation" model using multiple methods and instruments to compare two versions of a science curriculum and embedded assessment system. We present results from our validation study and discuss the challenges in determining the extent to which teachers use a curriculum as designed. We focus as well on the practical curriculum implementation issues amidst the needs and perspectives from different stakeholders. Appended are: (1) Magnetism and Electricity Module: Pre/post Assessment; (2) Teacher Content Survey; (3) ASK/FOSS Classroom Assessment Observation Protocol; (4) ASK Study Research-Phone Interview; (5) FOSS Study Research-Phone Interview; (6) Weekly Teacher Log-FOSS/ASK Water Module; and (7) Weekly Teacher Log-FOSS Water Module. (Contains 16 tables and 2 figures.)