The value and effectiveness of formative assessment in the classroom has gained an increasing amount of attention during the past decade, especially since the publication of seminal work by Black and Wiliam titled "Assessment and Classroom Learning." Since that time, there has been a renewed interest in describing and evaluating teacher practices related to formative assessment. Based on evidence of its effectiveness in the classroom and on improving standardized test scores, many prominent educational entities have initiated reform efforts to promote the use of formative assessment, yet these practices have not been embraced by classroom teachers. This case study investigated internally constructed and externally imposed contextual elements that constrained or facilitated the use of formative assessment by three high school science teachers. Cornett's curriculum development model of personal practice theories was modified to include assessment, termed "personal practice assessment theories" (PPATs), and chosen as a framework for the study. This research revealed distinct differences among the three teachers' PPATs and several different factors that constrained or facilitated the use of formative assessment in their instruction. Most notable of these factors were the forms of teacher knowledge that played a critical role in shaping their assessment practices and had a bearing on their ability to convert espoused theories about assessment into actual classroom practice. Other externally imposed barriers that constrained the use of formative assessment included expectations, habits, and dispositions of students; the pressure that teachers felt to "cover" all of the curriculum in order to prepare students for the end-of-year, high-stakes exam; and an instructivist rather than constructivist approach to teaching and learning. Results from this study add to the growing body of knowledge about the complex terrain teachers negotiate in making teaching and assessment decisions and provides a framework for future studies.