This paper is the third in a series concerning English language proficiency (ELP) assessments for K-12 English learners (ELs). The series, produced from Educational Testing Service (ETS), is intended to provide theory- and evidence-based principles and recommendations for improving next-generation ELP assessment systems, policies, and practices and to stimulate discussion on better serving K-12 EL students. The first paper articulated a high-level conceptualization of next-generation ELP assessment systems (Hauck, Wolf, & Mislevy, 2016). The second paper addressed accessibility issues in the context of ELP assessments for ELs and ELs with disabilities (Guzman-Orth, Laitusis, Thurlow, & Christensen, 2016). The fourth paper dealt with one of the major uses of ELP assessments--the initial identification and classification of ELs (Lopez, Pooler, & Linquanti, 2016). The present paper focuses on summative ELP assessments that states use for accountability purposes. Based on the conversation that took place at the K-12 ELP Assessment Working Meeting on summative ELP assessments, we identify critical policy and research issues. We also highlight the challenges associated with measuring ELP in a diverse group of EL students and recommend ways to embrace and support EL students through shared goals and active communication across federal agencies, state agencies, local agencies, researchers, and practitioners.