Background: When New York City's (NYC) Division of Early Childhood Education (DECE) enacted their historic expansion of free programming for four-year-olds under Pre-K for All, they designed a complex system of professional learning (PL) to support teachers and high-quality programming. This study, developed in partnership between NYU and DECE, evaluates the impact of two PL "series" provided to teachers in 2021-2022, the year after the pandemic. Each PL series had a content-specific focus, relative to a third content-agnostic PL series. The first PL series, Explore, combines an evidence-based math curriculum known as Building Blocks (Clements & Sarama, 2008; Sarama et al., 2008) with research-based interdisciplinary units developed by the DECE to embed math learning in other areas (e.g., social-emotional development and literacy). The second, Create, is an arts-based approach with the goal of incorporating visual arts, dance, theater, and music into ongoing instruction to promote learning across domains. The goal is for teachers to provide children with the opportunities and raw materials to express themselves, explore new concepts, exercise creativity, and collaborate with teachers and peers. Teachers are provided with different personalized supports by onsite coaches and community-based experts in creative arts, respectively. The Inspire track, also called TTLC, provides professional learning sessions tailored to the needs of pre-K teachers and leaders, and include a variety of topics aligned to the district's quality standards that support early child instructional goals. Given its focus on meeting a wide range of teacher and leader needs and emphasis on providing sites more choice in the focus areas of particular sessions, it has much less clear teacher practice targets, on average, and thus there is a less clear expected theory of change. Professional development series aim to provide teachers with tools to support children's needs and development in their classrooms, and in Pre-K for All, they are meant to ensure high-quality teaching. Yet, in a post-pandemic context, the role professional development series could play in supporting early childhood teachers is unclear. The COVID-19 pandemic led to several disruptions in young children's lives and their families (Patrick et al., 2020), alongside exceptional challenges for early childhood teachers (e.g., Crawford et al., 2021), likely leading to important implications in children's developmental pathways. This study aims to fill that gap. Methods: We leverage the city's randomization of pre-k sites to series (i.e., Explore, math-focused; Create, arts-focused; Teaching Team Learning Communities (TTLC), content-agnostic) in the 2021-2022 school year to examine the impacts of Explore and Create relative to a content-agnostic series (TTLC). We hypothesize that a) content-specific PL will impact teaching practices, and b) teaching practices will be associated with aligned child outcomes. We present findings regarding both hypotheses in this presentation. The city collected site preference with a survey. Explore and Create each had a capacity limit, and when preference exceeded availability in that PL series, sites were randomly assigned to the PL series they preferred or to TTLC (the control group). Create had capacity to serve 36 programs yet 308 ranked it first, resulting in 36 randomized to Create and 272 to TTLC; Explore had capacity for 50 programs yet 217 ranked it first, leaving 50 randomized to Explore and 167 to TTLC. The NYC DECE conducted observations of pre-k programs as part of their quality monitoring, using standard protocols (Classroom Assessment Scoring System (CLASS) and the Adapted Teaching Style Rating Scale (A-TSRS)), in 543 classrooms across 188 randomized sites. NYU completed direct assessments of children's skills (e.g., math, language, executive functioning) in the spring of pre-k with 799 children across 85 recruited sites from those randomized. Our sample is socioeconomically and racially diverse, reflecting NYC's diversity. Results: We found randomization was maintained across sites and children in our sample across all treatment/control contrasts. Analyses are underway to assess the impacts of PL on teaching practices, using separate analyses for: (a) Create vs TTLC, (b) Explore vs TTLC, (c) content-specific (combined Create/Explore) vs content-agnostic (TTLC). Next, we will assess the impacts of teaching practices on aligned child outcomes (e.g., teachers' math practices and children's math abilities). We use multilevel modeling for our nested data (teachers or children in sites). Preliminary impact analyses show few impacts of content-specific PL on teacher practices and children's outcomes. Explore, relative to TTLC, improved quantity and quality of math instruction, as expected; yet, surprisingly, we found null impacts on children's math learning. By contrast, Create, relative to TTLC, had null effects on measured classroom practices, yet demonstrated a number of benefits to child outcomes, including language, literacy, and math scores. Further analyses will be conducted to understand heterogeneity of impacts across sites and impacts on classroom practices are related (or not) to children's outcomes. Implications: NYC is the largest, most diverse district in the nation, serving 1.1 million students overall and [approximately]70,000 pre-k students in [approximately]1,800 programs. This provides a unique opportunity to examine pre-k at scale for a diverse population. By evaluating NYC's complex system of PL and how teaching practices are associated with children's outcomes, we provide vital information to city leaders implementing and strengthening universal pre-k systems. To fulfill the promise of pre-k, knowledge of how PL can promote high-quality teaching practices and children's outcomes at scale is critical, especially in a post-pandemic context.