With the United States' adoption of a standards-based approach to education, most attention has focused on the large-scale, high-stakes assessments intended to measure students' mastery of standards for accountability purposes. Less attention has been paid to the role of standards-based assessments in the classroom. The purpose of this paper is to discuss key issues and challenges related to the use of standards-based classroom assessments to assess English language learners' English proficiency. First, the paper describes a study of a standards-based classroom assessment of English proficiency in a large urban school district in California. Second, using this study as an example and drawing from the literature in language testing on classroom assessment, this paper highlights the major issues and challenges involved in using English proficiency standards as the basis for classroom assessment. Finally, the article outlines a research agenda for the field given current developments in the areas of English proficiency standards and classroom assessment.