According to the Michigan Department of Education, between 2011 and 2017, the number of multilingual learners (MLs) enrolled in K-12 schools in the state increased by more than 50%. In 2017, the average teacher to ML ratio in the state was an alarmingly low 1:168. To increase the number of teachers prepared to effectively serve MLs, we developed Project English Learners and Teacher Education (ELATE), a professional development program grounded in adult learning theories and centered around the Sheltered Instruction Observation Protocol (SIOP). SIOP is a research-based instructional framework that aims to prepare teachers to bridge the gap between language and content instruction for MLs. It has the following eight components: lesson preparation, building background, comprehensible input, strategies, interaction, practice/application, lesson delivery, and review and assessment. Limited empirical research suggests that SIOP is a reliable tool for detecting instructional intervention effects. Therefore, the purpose of this study was to (1) add to the empirical research focused on SIOP and (2) determine the impact of an intensive professional development program on in-service teachers' use of instructional practices for MLs. We implemented a quasi-experimental study focused on Project ELATE, a 5-year National Professional Development (NPD) program that trained 170 in-service teachers in providing quality instruction for MLs. The primary goal of Project ELATE was to improve in-service teachers' knowledge and instructional skills for supporting and teaching MLs. Training included graduate courses, clinical experiences, video demonstrations, a conference, and instructional coaching. Data sources included pre- and post-program observations, interviews, and written reflections. Results showed statistically significant SIOP score increases for participants from pre- to post-program on all eight components, while controls showed no significant increases. Overall, the findings provide evidence that Project ELATE significantly improved teachers' ability to use sheltered instructional practices. Qualitative data highlighted that participants benefited from feedback from multiple sources, including an English as a Second Language coach, video observations, and professional learning communities. In summary, this NPD program demonstrated success in improving teachers' instructional practices for MLs.