With the expansion of NYC’s universal pre-K in 2014 in Pre-K for All (PKA), the NYC Department of Education tackled the challenge of supporting classroom quality through professional development (PD) across 2,500 classrooms and a large, rapidly expanding workforce. We conducted analysis with administrative data and newly collected surveys and interviews to understand mechanisms for strengthening classroom quality via the distribution of ECE teachers across the city and support of teachers’ professional learning through district-led workforce development and site-level advice networks. (author abstract)