Formative Instruction is notoriously difficult for teachers to implement. I characterize Formative Instruction as a cognitive, technological, and organizational practice, describe a technology-supported, cross-cutting intervention in teachers' practice that responds to these dimensions, and make predictions about how results of the intervention may vary in accord with different sets of teacher beliefs. I describe a case study of teachers attempting to improve their content area instruction by using web-based tools I created to assist them in analyzing students' annotations, which I argue can be useful representations of students' thinking. My results include an elucidation of how specific teacher beliefs and past instructional practices were related to teachers' choices about how to adapt annotation, to analyze students' work, and to reason about student thinking for ongoing instruction. I use my results to describe ways to better theorize about school reform and establish stronger researcher-practitioner collaborations. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]