Research on mathematics classroom learning opportunities shows that student's experiences with mathematics affects not only what they learn, but also how they view themselves as learners and doers of mathematics (Boaler, 1997, 2016; Jackson, 2009; Langer-Osuna, 2016). High-stakes testing policies have implications for the kinds of mathematics instruction students receive given that teachers' sense-making and instructional decisions can vary based on state and local policies (Nelson et al., 2012; Bertrand & Marsh, 2015; Diamond, 2007; Spillane et al., 2002). The norms and activities that teachers plan are ultimately informed by the ways that teachers make sense of these policies and how their decisions translate into practice. Opportunities to learn arise from these norms and activities that teachers cultivate in the classroom and shape how students engage with mathematics (Gresalfi & Cobb, 2006; Greeno & Gresalfi, 2008). Yet, little work has focused on the relationship between testing, classroom practices, and, in particular, how teacher sensemaking of high-stakes testing policies enable or constrain opportunities to learn for students. This dissertation seeks to provide insight into the multilayered relationship between educational policies and students' mathematical identity development by highlighting the role that teachers play in making sense of and drawing on policies to shape mathematical learning opportunities in 3rd and 4th grade classrooms. By engaging the analytic lens of teacher sensemaking, this qualitative study centers the voices of teachers broadly across California and Texas as a means to understand how they made instructional decisions when federally mandated assessments were introduced. This study further provides deeper insights into how teachers' sense-making was translated into practice and how students perceived the learning opportunities in the classroom to inform their mathematical dispositions and conceptions of mathematical identity. My first findings chapter highlights that there were clear differences in the knowledge that teachers had about federal, state, and district policies. This knowledge informed what teachers reported needing to focus on for their decisions for teachers in Texas, and a lack of knowledge provided opportunities for teachers in California to exercise more agency in their instruction. Even though almost all teachers reported feeling that policies focused on assessments had negative consequences such as narrowing their instructional focus and the effect on student morale, they still felt that assessments provided necessary data for holding teachers and students accountable. For teachers in dual-language or bilingual classrooms, the varying policies that teachers navigated highlights the complex practice of sense-making that teachers further engaged in. Teachers felt constrained when the language of assessments differed from the language of instruction. Additionally, when so much emphasis continues to be placed on English-language proficiency, teachers unintentionally reify negative language ideologies for linguistically diverse students. My second findings chapter centers around two teachers within one California school district. While the teachers highlighted in this chapter spoke to a focus on district benchmarks as important for accountability, they remained committed to their own educational philosophies and engaged in student centered practices. The teachers' ability to construct lessons centered around providing opportunities to learn (Gresalfi & Cobb, 2006) was reflected in the learning opportunities students reported. The dual-language teacher embraced translanguaging practices, shaped by her own experiences as an emergent bilingual. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]