This study examined the impact of teachers' characteristics and self-reported practices on students' Algebra achievement while controlling for students' characteristics. This study is based on the secondary analysis of data collected from a nationally representative sample of 9 th grade students and their mathematics teachers during the base year of the High School Longitudinal Study of 2009 (HSLS). Student and teacher data were obtained through questionnaires and students' Algebra achievement was measured by an ability-adaptive assessment designed to measure a cross-section of understandings representative of the six major Algebraic content domains (The language of algebra; Proportional relationships and change; Linear equations, inequalities, and functions; Nonlinear equations, inequalities, and functions; Systems of equations; and Sequences and recursive relationships) and the four major Algebraic processes (Demonstrating algebraic skills; Using representations of algebraic ideas; Performing algebraic reasoning; and Solving algebraic problems). For the HSLS data were collected from different participants at the beginning, middle, and end of the school year. Four linear regression models were used to investigate the significant predictors of students' Algebra achievement. Model 1 included only students' characteristics. Model 2 built on model 1 by adding only teacher characteristics. Model 3 also built on model 1 by adding only teachers' self-reported practices. Finally, model 4 built on model 1 by adding both teachers' characteristics and self-reported practices. The models were run separately based on whether the participants were members of the beginning, middle, or end of the school year groups. Results indicated that model 4 explained the most variance in students' Algebra achievement followed by model 2, model 3, and then model 1. Analyses of the regression models revealed that teachers' characteristics and self-reported practices each individually impact students' achievement while controlling for students' characteristics; teachers' characteristics has more of an impact on students' achievement than their self-reported practices; and teachers' self-reported emphasis on Standards-based objectives explains a unique and significant portion of the variance in students' achievement controlling for students' and teachers' characteristics. Results also imply that impact of teachers' characteristics and self-reported practices increases over time. Within each model findings indicate that several student- and teacher-level variables may have a significant impact on students' Algebra achievement. Regarding the students' characteristics included in models 1--4, students' prior curriculum studied, prior achievement, SES, identity, self-efficacy, interest, and attending private school exhibited only positive effects; being Black and students' math utility only showed negative effects; and being female, Hispanic, and attending a private school demonstrated both negative and positive effects. Concerning the teachers' characteristics included in models 2 and 4, teachers' years of experience and beliefs about students' abilities exhibited only positive effects; teachers' self-efficacy showed only negative effects; and teachers' certification showed mixed effects. Considering the teachers' self-reported practices included in models 3 and 4 emphasizing standardized test preparation, the logical nature of math, preparing for future math study, and teaching concepts only exhibited positive effects; focusing on computational skills, algorithms and procedures, and problem solving skills only demonstrated negative effects; and emphasizing effectively explaining math ideas, increasing interest, speedy and accurate computations, reasoning skills, the history and nature of math, and business and industry applications showed mixed effects. Furthermore, results indicate that teachers' characteristics and self-reported practices may explain part of the reason "why" students' characteristics impact students' achievement; teachers' self-reported practices may explain part of the reason why teachers' characteristics impact students' achievement; and that time may mediate the influence of certain predictors on students' achievement. The final analyses revealed that a substantial proportion of secondary mathematics teachers reported placing moderate to heavy emphasis on Standards-based instructional objectives that demonstrated a significant "negative" effect on students' achievement in at least one model. Furthermore, a noteworthy percentage of secondary mathematics teachers reported placing no to minimal emphasis on Standards-based instructional objectives that displayed a positive effect on students' achievement in at least one model. [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.]