As early education grows in the United States, in-service professional development in key instructional and interaction skills is a core component of capacity building in early childhood education. In this article, we describe results from an evaluation of the effects of MyTeachingPartner, a web-based system of professional development, on language and literacy development during prekindergarten for 1,338 children in 161 teachers' classrooms. High levels of support for teachers' implementation of language/literacy activities showed modest but significant effects for improving early language and literacy for children in classrooms in which English was the dominant language spoken by the students and teachers. The combination of web-based supports, including video-based consultation and web-based video teaching exemplars, was more effective at improving children's literacy and language skills than was only making available to teachers a set of instructional materials and detailed lesson guides. These results suggest the importance of targeted, practice-focused supports for teachers in designing professional development systems for effective teaching in early childhood programs. (Contains 3 tables and 1 figure.)