This phenomenological study describes how secondary mathematics preservice teachers (PSTs) in a teacher residency program designed and implemented project-based learning (PBL) units. The Six A's, a framework used to evaluate the rigor and relevance of PBL units, helped capture the research question: What are the successes and challenges PSTs experience as they implement PBL units for the first time? This qualitative study illustrates how PSTs engaged students in relevant learning, wrestled with sustaining the rigor, and included the community to be partners in the projects. Findings indicate that high-quality implementation of PBL instruction requires involving the community, facilitating mathematical learning, and transforming the learning environment. PSTs recognized that a PBL approach requires a shift from traditional teaching practices and reconceptualizing both what it means for teachers to teach mathematics and for students to learn mathematics. This study contributes to the scarce body of knowledge on how teacher residency programs can utilize PBL as an instructional model to prepare PSTs for PBL environments.