Currently, states are moving forward with developing or adopting new English language proficiency (ELP) assessments aligned with the new academic standards in order to meet accountability requirements. In this transition, it is essential to identify areas of improvement for current ELP assessment practices and discuss ways to implement effective ELP assessments in order to better serve the needs of English language learner (ELL) students. This article offers a conceptual framework and guiding principles to consider in designing and developing next-generation ELP assessment systems for K-12 ELL students. In particular, the article calls for a need to create an ELP assessment system where the standards, a set of coherent assessments, and instruction are all systematically linked to maximize assessment results and thus aid in ELL students' academic success. The importance of defining an overarching construct to govern various assessments and the use of evidence-centered-design principles for the development of an assessment system are also discussed.