Next Generation Science Standards (NGSS) science and engineering practices (SEPs) involve commands of the English language in writing, speaking, reading, and listening. Language tasks embedded in NGSS SEPs require English learners (ELs) to develop science skills and understanding and language proficiency simultaneously. Sense of self-efficacy about teaching science with ELs plays a critical role in teachers' practice in supporting ELs. This convergent mixed-method study examined middle school science teachers' implementation of language-based strategies in NGSS classrooms. Specifically, the study sought to answer: (1) What language-based strategies do middle school science teachers implement in teaching the NGSS curriculum?, (2) What SEP-aligned language tasks do teachers engage ELs while implementing the language-based strategies?, (3) Which of the language-based strategies do teachers implement to support ELs in meeting the language demands in SEPs?, and (4) Is there a significant relationship between middle school teachers' self-efficacy beliefs about teaching science to ELs and implementation of language-based strategies in science classrooms? Researcher-developed instruments, including middle school science teachers' language strategies survey and modified self-efficacy beliefs about teaching science with ELs scale and interviews were used to collect data. Middle school teachers implement language-based strategies at varying levels in NGSS-based classrooms. The most frequently implemented strategy is collaborative grouping while the least implemented are engaging students in oral discourse and use of performance-based tasks. Teachers engage ELs more with SEP-aligned language tasks that use and practice the English language (receptive and productive tasks) than the language tasks (analytical) for scientific understanding while implementing language-based strategies. Teachers implement use of notebooks, scaffolding techniques, technology tools, and collaboration to support ELs in accessing the language demands in doing SEPs. Finally, there is a significant relationship between teachers' integration of language-based strategies and self-efficacy beliefs about teaching science with ELs. Findings of this study contribute to the body of research on teachers' practices with ELs in the science classrooms while implementing the NGSS; however, this conclusion must be treated with caution due to the limited number of participants in this study, that data collection was based on self-reporting, and the study was conducted during the pandemic crisis. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]