In K-12 environments in the US, classroom tests are a central means by which teachers assess English Language Learner (ELL) content knowledge. Performance on routine classroom assessments is often a contributing criterion for school based decision-making and can affect decisions relating to academic tracking, retention, and access to academic pathways. This mixed-methods study investigates how content teachers assess ELLs, what beliefs guide their practice, and how ELLs perceive the content assessments that they take. In this study, elementary math and science teachers (n = 213) in ten Pennsylvanian school districts were surveyed to quantitatively analyze assessment practices for ELLs. Observations of everyday math and science tests were conducted in ten 4th grade classrooms, followed by interviews with ten classroom teachers and fifty 4th grade ELLs. Survey findings revealed high rates of teacher uncertainty of ELL proficiency levels. Classic accommodations of additional time and teacher assistance were reported to be most commonly implemented in K-6 classrooms, and English dictionary use and all native language accommodations were rarely implemented. Focal findings identified twenty-one assessment practices and four accommodated scoring practices in place for ELLs during math and science assessments. Observations and interviews found that accommodations implementation varied widely according to teachers' perceptions of students' needs. Evaluation of language proficiency was generally guided by teacher judgment of students' ability to participate in classroom tasks rather than by determined proficiency levels. Beginners who had the greatest difficulty participating meaningfully in classroom assessment typically received the most accommodations. Accommodated scoring practices were commonly implemented based on attention or effort, and ELLs reported a general lack of understanding of these grading systems under which their work was evaluated. ELLs represented themselves as liking tests since tests offered them opportunities to learn, though they reported some feelings of anxiety when completing timed test tasks. An overarching finding of this study was that ELLs were found to be systematically placed in low tracks of math and science based on high-stakes test performance. This study represents a starting point from which to consider classroom content assessment for ELLs and raises important questions about the validity of their classroom test scores in school-based decision making. [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/en-US/products/dissertations/individuals.shtml.]