This study evaluated the efficacy of the Word Learning Strategies (WLS) supplementary program to improve elementary students' vocabulary skills and reading comprehension. The study used a multi-site cluster randomized, experimental design, which randomly assigned 92 4th grade classrooms (n=2558 students) from two cohorts to a treatment or control group. Results indicated that the program was positively associated with gains in students' vocabulary learning and knowledge as measured by the Word Learning Strategies Test and the VASE Assessment, and in students' reading comprehension as measured by the Gates-MacGrinitie Reading Test, after accounting for differences in baseline measures. The use of the WLS program also led to increases in teachers' awareness of strategies to support their students' vocabulary and reading comprehension. [This paper was published in the Proceedings of the 2019 Annual Meeting of the American Educational Research Association (AERA), Toronto, Canada.]