Background/Context: Professional development (PD) programs have been the primary tool school districts have used to improve teachers' knowledge and skills, though the evidence is mixed on the degree to which these investments translate into improved outcomes for teachers and their students. Further, most research has tracked researcher-designed and researcher-implemented programs, meaning we know far less about the outcomes of PD designed and implemented by districts. Given that implementation and associated outcomes may look different without tight research parameters, we need more systematic research about district-designed and implemented PD. During early years of PD implementation, it is more likely to observe changes in more proximal outcomes, including an increased sense of trust and collaboration with colleagues, which could, in turn, support teacher retention. Any intervention, but especially those that necessitate substantial changes in instructional activities, likely takes time to promote changes to downstream outcomes like high-stakes assessments of teaching and student achievement. Purpose/Objective/Research Question/Focus of Study: We analyze the relationship between the design and implementation of an ambitious PD/professional learning (PL) program, called Learning Together to Advance Our Practice (LEAP), and a range of outcomes across 3,000 teachers in the District of Columbia Public Schools (DCPS). We examine the extent to which teacher-reported frequency of participation in two specific PD structures—one-on-one coaching and team seminars—are each associated with improved outcomes of interest. Proximal outcomes include teacher perceptions of the PL program and peer culture at their school, as well as school- and district-wide retention. More distal measures include teacher classroom practice and student achievement. Research Design: We capitalize on researcher-designed and district-administered survey questions, along with rich administrative data, to understand the relationship between this at-scale, intensive PL program and a range of outcomes over two years, from 2016 to 2018. DCPS implemented LEAP simultaneously in every school at the beginning of the 2016–2017 school year. As a result, our ability to identify how our outcome variables would have changed in the absence of LEAP is limited. We address this issue by measuring differential implementation because the frequency of teacher participation in LEAP varied within schools, within LEAP teams within a year, or within a teacher across a two-year period. We hypothesize that more exposure to LEAP yields greater improvements in outcomes. In separate models, we attempt to limit competing explanations by controlling for: (1) observable attributes of teachers and time, and unobservable, time-invariant attributes of schools; (2) unobservable, time-invariant attributes of LEAP teams; and (3) unobservable, time-invariant attributes of teachers. Conclusions/Recommendations: We find that greater reported engagement with LEAP is associated with improved teacher perceptions of LEAP and the peer culture at their school, as well as improved teacher retention, especially at the school level. This suggests that PL programs that center within-school connections and supports for teachers—in this case, vertically structured LEAP teams led by school-based LEAP leaders—may support positive school-level outcomes. However, we find little evidence of improved teacher skills and teacher contributions to student achievement, at least in the first two years of LEAP implementation. It will take more time and research to understand the degree to which and ways in which the district's investment in LEAP is associated with the range of desired outcomes. [ABSTRACT FROM AUTHOR]