We assessed the relationship of level of QualityCore[R] implementation and student achievement in Algebra I. The study is based on 1,291 9th grade students in three large metropolitan school districts who took EXPLORE in the fall and the QualityCore Algebra I end-of-course assessment (EOC) in the spring. During the same period, the students' 41 Algebra I teachers completed questionnaires measuring their use of QualityCore's tools and strategies for Algebra I. We found that level of teachers' QualityCore implementation had a positive and statistically significant relationship with student achievement, measured by performance on the QualityCore Algebra I EOC controlling for performance on the fall EXPLORE tests. When different domains of implementation were considered, the "Teaching Practices and Collaboration with Colleagues" domains had the strongest relationship with student achievement. Survey results also indicate how QualityCore use varied across components and that QualityCore was well-received by teachers. Appended are: (1) Survey Items and Statistics; and (2) Results of Hierarchical Linear Models. (Contains 13 tables and 1 footnote.)