According to the vision of the "Next Generation Science Standards" (NGSS), science and engineering are language intensive and offer both opportunities and challenges for all students, especially English learners (ELs), who are developing language and disciplinary knowledge and practices simultaneously. To support ELs of varying levels of English proficiency, the teacher must gather information about student learning that can be used to advance students' language proficiency in science. Formative assessment can be a powerful means for gathering such information and using it to provide ongoing and contingent support. This article focuses on formative assessment of language proficiency (shortened to "proficiency") in the context of NGSS-aligned science instruction. This article provides ideas and techniques to enhance science teaching.