How can formative assessment enhance the teaching and learning of English language learner (ELL) students? What, if anything, from our experience with summative assessment of ELL students can inform effective formative assessment practices? And finally, what are the opportunities and challenges inherent in integrating formative assessment into instruction for ELL students in this era of Common Core and other next generation standards? This paper addresses these questions. In addition, the authors, all former or current WestEd researchers, argue that in order to use formative assessment effectively in classrooms with ELL students, teachers must attend simultaneously to the students’ needs both in learning content and skills, as well as in developing the English required to express their learning.