This mixed method study explored and analyzed instructional strategies utilized by algebra teachers whose students' coursework culminated in the New York State Regents Examination in Integrated Algebra and for whom 50% of the tested cohort earned mastery level (85 or higher) on the examination. The targeted populations were eighth or ninth grade students in suburban school districts in New York State. The researcher employed a survey tool consisting of a four-point Likert scale with a double endorsed component that was completed by algebra teachers, along with seven qualitative interviews of direct supervisors of mathematics instructional programs. Results of the survey, student performance (as reported on the New York State Report Card), and interviews were analyzed and coded utilizing a one-way ANOVA and MANOVA. Overall student performance and the survey results were analyzed using data collected from teacher surveys and district performance on the New York State Regents Examination in Integrated Algebra. The interview responses were coded and analyzed. Results revealed significance between teachers' reported levels of expertise in differentiated instruction, in both belief and practice, in the areas instructional strategies, students interests, co-teaching models, and classroom supports and aids. Results of qualitative data revealed patterns of strong vision and commitment by supervisors in the areas of the importance of algebra, instructional strategies that foster algebraic achievement, programmatic supports, and overall teacher support and staff development. [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.]