This professional paper builds on the work of TESOL's issue briefs, "Implementing the Common Core State Standards for English Learners: The Changing Role of the ESL Teacher" (February 2013) and Overview of the Common Core State Standards Initiatives for ELLs (March, 2013). It discusses the shifting landscape surrounding the new Common Core Standards and the Next Generation Science Standards and their implications for building and enacting teacher expertise. This paper is written for K-12 ESL professionals, including teachers, teacher-leaders, school principals, district administrators, and other K-12 educators who work primarily or exclusively with English language learners. This document includes the appendix: "Historical and Current Conceptualizations of Language and SLA in Language Teaching: A Basis for Rethinking." [For "Overview of the Common Core State Standards Initiatives for ELLs. TESOL Issue Brief," see ED560133 ; for "Implementing the Common Core State Standards for English Learners: The Changing Role of the ESL Teacher. A Summary of the TESOL International Association Convening April 2013," see ED560134.]