English learners (ELs) constitute one of the largest subgroups enrolled in the US public education system. The state of California has an estimated 1.5 million ELs in kindergarten through Grade 12. This mixed methods study employed improvement science principles to understand how three high school physics teachers implemented project-based learning (PBL) as an approach toward improving science instruction for ELs. This dissertation study addressed the following questions: What benefits do ELs obtain from experiencing PBL? What is the effect of teachers' mindsets on student achievement in PBL-driven classrooms? How can teachers use PBL to engage ELs effectively in learning science content and practices? How do teachers leverage the assets of ELs when they develop and implement PBL lessons? Results indicate that PBL has provided significant benefits to ELs in the areas of academic engagement, mindset, confidence, sense of belonging, personal accountability to learning, and problem-solving skills. Additionally, the participating teachers' use of PBL has led to meaningful changes in their perceptions on curriculum design and planning for ELs. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]