The purpose of this study was to examine teachers' perceptions of the Illustrative Mathematics (IM) using data from teacher interviews from year 1 and year 3 of their implementation of IM. In year 1, 22 Algebra 1 teachers were interviewed; in year 3, 13 teachers participated. Interviews were conducted using a semi-structured protocol that included open-ended questions as well as more specific questions. Coding involved three aspects of IM (real-life contexts, rigor, conceptual understanding), resources, and teacher perceptions. Coded comments were then labeled as positive, negative, or both and patterns across both years of data collection were compared. The results indicated that some themes were consistent throughout the three-year span (benefits of IM's real-world applications, challenges with sufficient time to implement IM as designed). Other themes changed, including teachers appreciating the ready-to-use IM materials in year 1 shifting to an appreciation for the design of the curriculum (which included spiraling attention to concepts) in year 3. As increasing numbers of districts move to open-source curricula like IM, the results of this study hold implications for curriculum designers, researchers, and district decision makers, expanding knowledge of how teachers' perceptions of the curriculum over time. [This paper was published in: "Proceedings of the Forty-Fourth Annual Meeting of the North American Chapter of the International Group for the Psychology of Mathematics Education," edited by Alyson E. Lischka et al., Middle Tennessee State University, 2022, pp. 247-248.]