In 1998 Paul Black and Dylan Wiliam published the article, "Inside the Black Box: Raising Standards Through Classroom Assessment" (Black & Wiliam, 1998b). They asserted that formative assessments were the strongest way of raising student achievement. There are a number of empirical studies that document positive impacts of formative assessment on student learning (Brookhart, 2004; Allal & Lopez, 2005; Koller, 2005; Brookhart, 2007; Wiliam, 2007; Hattie & Timperley, 2007). There are also critics of much of the existing research (Shute, 2008; Dunn and Mulvenon, 2009; Bennett, 2011; Coffey Hammer Levin and Grant, 2011). The literature points to the need for more research in this area. Shavelson (2008), who looked at formative assessment in the science classroom stated, "[a]fter five years of work, our euphoria devolved into a reality that formative assessment, like so many other education reforms, has a long way to go before it can be wielded masterfully by a majority of teachers to positive ends. This is not to discourage the formative assessment practice and research agenda." In this study I examined how teachers understand formative assessment in relation to their instruction, and how they actually implement formative assessment in their math classrooms. I used a thematic analysis research design, analyzing interviews, and observation recordings and field notes. I created a Depth of Implementation Framework, based on both a definition of formative assessment constructed from the review of literature and on the data gathered from teachers, to help make sense of the interplay between teachers' understanding and use of formative assessment. Based on the data, teachers' use of formative assessment was characterized as deep, developing or superficial. Teachers' understanding of formative assessment, especially the definitions that they constructed for themselves, had an impact on both how they used formative assessment and how they saw themselves improving their use of formative assessment. [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.]