Assessment of and for learning has occupied center stage in education reform, especially with the advent of the No Child Left Behind Federal legislation. This study examined the formative function of assessment--assessment for learning--recognizing that such assessment needs to be aligned, at least in part, with the summative function of assessment--indexing achievement against standards and progress. Black and Wiliam (1998) suggested that formative assessment might very well improve student learning. Based on these ideas and our own experience with reform science education, we hypothesized that for a small investment of resources we might have a major impact on achievement by embedding formative assessments in a nationally used curriculum. To this end we created a collaboration, described here, between curriculum and assessment developers, created embedded, formative assessments, and studied the impact of science teachers teaching with these materials on middle-school students' motivation, achievement, and conceptual change in a small randomized trial. We also studied the collaboration itself with the intent of informing others who might wish to enter into such collaboration about the potential strengths and challenges experienced. The articles that follow in this special issue report in detail what we did and found out; this article provides a rationale and overview for the study. (Contains 3 figures, 1 table and 3 footnotes.)