Using a research design that meets the Institute of Education Sciences (IES) What Works Clearinghouse evidence standards, researchers reported the results of analyses examining the impact of the Teacher Study Group (TSG) professional development (PD) program targeting eighth grade American History teachers working with a large number of current or former English learners. Researchers reported statistical significance and effect sizes of the PD impact on teacher outcomes (i.e., observed vocabulary instruction) and on student outcomes (i.e., academic vocabulary and content knowledge). The TSG PD program resulted in significant impacts at the teacher level on observed teaching practice as measured by "Observation Measure for Vocabulary Instruction" ("OMVI"; g = 0.809; p = 0.0001). Specifically, the Hedges' g for two "OMVI" subscales was 0.926 (p < 0.0001) on the "Teacher-Directed Vocabulary Instruction" and 0.650 (p = 0.0021) on the "Interactive Vocabulary Instruction Scale." There were, however, no impacts at the student level for the main student sample or for the ELL subsample on the Academic Vocabulary measure or the "Assessment of Social Studies Knowledge (ASK) Content Knowledge" measure. Effect sizes ranged from 0.02 to 0.07 and were nonsignificant.