Chicago Public Schools is working to improve early literacy outcomes through a multiyear professional development initiative for preK-2 teachers. The P-2 Balanced Literacy Initiative aims to improve literacy instruction by training teachers to implement effective early literacy instruction balancing systematic foundational skills instruction with reading and writing instruction involving rich, complex texts. The initiative began in 2016/17 and served 23 percent of all district elementary schools by 2018/19. The district designated 26 of the 115 elementary schools implementing the initiative in 2018/19 to receive enhanced supports, including intensive, site-based coaching, to support students' independent reading. This study compared the reading achievement of students who attended schools that received the enhanced supports (priority schools) with the reading achievement of students who attended similar schools that received only the initiative's standard supports (nonpriority schools). It also examined differences between priority and nonpriority schools in teachers' and administrators' participation in professional development sessions and looked at the successes and challenges of implementation. The study found that one year after implementation of the initiative, attending a priority school did not lead to higher end-of-year reading achievement than attending a nonpriority school after other factors were adjusted for. Teachers and administrators in priority schools were more likely than those in nonpriority schools to participate in the initiative's core professional development sessions. Interviews with select district, network, and school leaders; instructional support coaches; and teachers suggest that several aspects of the initiative's professional development were valuable, most notably the opportunities for teachers to deepen their understanding of the initiative's professional development, receive feedback through observation and school-based coaching, and learn from one another. But instructional support coaches' limited capacity, due to competing responsibilities, was a challenge. District leaders might consider increasing the number of coaches available and limiting their competing priorities so they can focus on the initiative. [For the Study Brief, see ED617590. For the Study Snapshot, see ED617589. For the appendixes, see ED617591.]