Reading comprehension seems to be a significant challenge for rising fifth graders with limited vocabulary knowledge. A possible solution is to provide them with explicit vocabulary instruction which may positively impact their reading comprehension. This action research aimed to evaluate the effect of implementing explicit vocabulary instruction delivered through Schoology on fifth graders' vocabulary knowledge and reading comprehension at an urban elementary school in the southeastern United States. This study also sought to analyze students' perceptions of the usefulness of the online vocabulary learning modules. A convergent mixed-method approach was applied to answer the research questions. The quantitative data (e.g., vocabulary and comprehension scores and the learner experience survey) used descriptive statistics, and the findings showed that the mean scores of the vocabulary post-tests significantly increased from the pretests. The pre-and post-test reading comprehension found that the post-test significantly increased from the pretests. In addition, the learner experience survey found that most participants considered instructional modules helpful for acquiring new vocabulary knowledge. Furthermore, an inductive approach analyzed the qualitative data gathered from semi-structured interviews. This data showed that even though explicit vocabulary instruction impacted students' vocabulary knowledge, there were areas in need of attention, such as the need for more explicit instruction in Latin and Greek roots. For elementary educators, practical implications on literacy education were the need to collaboratively design a vocabulary curriculum and explicitly teach vocabulary strategies to students. [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.]