The limited availability of research instruments that reflect the vision of the Next Generation Science Standards (NGSS) restricts the field's understanding of whether and how teachers are making instructional shifts called for by the standards. The need for such instruments is particularly urgent with teachers of multilingual learners (MLs), who are called on to make shifts in how they think about and enact instruction related to both science and language. The purpose of this study was to develop and gather validity evidence for a questionnaire that measures elementary teachers' beliefs, preparedness, and instructional practices for teaching NGSS science with MLs. We report on the development of the questionnaire over three phases that elicited multiple sources of validity evidence: (a) domain specification and expert review, (b) item writing and cognitive interviews, and (c) piloting and final item selection. Data included feedback from experts in science and language domains, cognitive interviews with 48 teachers, and a pilot with 310 teachers. Results indicated that the questionnaire differentiates among teachers with different levels of the underlying constructs and also that teachers' scores relate to their characteristics (e.g., familiarity with the NGSS). We highlight two implications for emerging research on NGSS‐based instrumentation: (a) the difficulty of communicating with teachers about science and language instructional shifts while teachers are still developing their understanding of such shifts and (b) the potential of emerging NGSS‐based instruments to inform professional development. We close with future directions for our research project specifically and the field of science education broadly.