This dissertation investigates and problematizes emergent bilingual students' achievement gaps in an existing corpus of middle-school science assessment data. I first characterize achievement gaps across national data, state data, and the corpus of approximately 27,000 students as an analytic backdrop. I then sample a subset of the 6th grade physics tests (n = 852) and employ computerized readability tools and multilevel modeling to determine whether linguistic complexity of item prompts predicts increased achievement gaps across emergent bilingual and native English speaking groups of students. Findings indicate that some measures of linguistic complexity are related to increased achievement gaps while others may serve to close achievement gaps. These results have important implications for the pressing debate surrounding language as a source of construct-irrelevant variance in content tests and test accommodations for emergent bilingual students (e.g. Abedi, 2006; Bailey, 2005; Farnsworth, 2008; Martiniello, 2008; Wolf & Leon, 2009). Motivated by results from the previous quantitative analysis, I conduct a follow-on qualitative study of 12 emergent bilingual and 15 native English speaking students' constructed responses in order to further explore sources of achievement gaps. Drawing on analytic tools from Interactional Sociolinguistics and schema theory, I show how the knowledge schemas (Tannen & Wallat, 1993) of each of the interlocutors involved in the test interactions--test developers, students, and raters--play a crucial role in the construction of meaning during the test taking process. Furthermore, I point to evidence of important contextualization cues that are used by the respective interlocutors to frame aspects of the test interaction as relating to certain target ideas. I also identify specific elements that contribute to schema miscues for emergent bilingual students, including the sequence of test items, the particular lexical items provided in the text prompts, grammatical aspects of test prompts, and visual accompaniment of items. Based on findings from the above analyses, I show that emergent bilingual students' "achievement gaps" are in fact--at least to a certain extent--created by monolingual content testing practices. Thus, I argue that monolingual content tests serve as gatekeeping encounters (Erickson, 1975; Erickson & Schultz, 1982) for emergent bilingual students. [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.]