Professional development (PD) is a potentially important mechanism for enhancing classroom practices and children's learning. In this large-scale randomized controlled trial, we examined the effectiveness of language and literacy PD, with and without coaching, offered at scale to early childhood educators (n = 546) across 1 state. Relative to the comparison condition, PD with coaching showed a small impact on the quantity of phonological awareness instruction, and PD with and without coaching impacted the quality of phonological awareness and writing instruction. PD did not impact children's (n = 1,953, M[subscript age] = 4.53) emergent literacy skills, as measured by the research team, or kindergarten readiness, as measured by the state's kindergarten readiness assessment which exclusively focused on language and literacy skills. Although we can only speculate as to why this at-scale, state-sponsored PD did not realize intended impacts, these findings, as coupled with those from the literature, raise critical questions concerning current understandings of PD and the ability to achieve desired effects when implemented at scale. [For the grantee submission, see ED597343.]