Imagine Math® is a standards-aligned, adaptive, interactive online mathematics program designed to supplement in-class instruction for students. This evaluation report examined the fidelity of implementation of Imagine Math® and the associations between its use and student achievement in Houston Independent School District (HISD) secondary schools in the 2018-2019 academic year. Based on teacher results, the research showed that the average levels of adherence to fidelity components were approaching expectations for both middle schools and high schools. At the teacher-level, frequency of use of Imagine Math® by teachers and analysis of the Imagine Math® reports for instructional purposes was low. Nevertheless, student progress and students' use of the program, were approaching expectations. Results of multiple linear regression analyses showed that, at the teacher-level, the Data Analysis component was a consistent statistically significant predictor of student achievement across levels. At the student level, however, the most important predictor of students' 2019 academic achievement was their previous year's STAAR scale score. The number of Imagine Math® lessons attempted with fidelity was also a statistically significant positive predictor of student achievement; the more lessons attempted the better the STAAR scaled score.