Developing teachers' knowledge about assessing English language learners (ELLs) is critical for achieving intended policy outcomes of improving teaching under Race to the Top policy and Common Core State Standards. In addition to teachers' expressed concerns about teaching Common Core State Standards to ELLs in a national survey published in "Education Week" (Gewertz, 2013), the formation of two assessment consortiums (i.e., Smarter Balanced Assessment Consortium, or SBAC, and Partnership for Assessment of Readiness for College and Careers, or PARCC) indicates the urgency for teachers to demonstrate their assessment knowledge and for schools to provide meaningful professional development opportunities in that area. In this article, we detail the rationale for school-based professional development targeting assessment, especially for new teachers, and outline a proposed procedure for providing such training.