Online professional development (OPD) has potential to improve teacher quality by improving teachers' knowledge and instructional practices. These changes, in turn, have potential to improve student achievement. Unfortunately, there is a dearth of scientific research on the effects of OPD on teachers and, more importantly, on students. This article presents the results of a randomized controlled trial exploring the effects of a series of three learning-community model OPD workshops on teachers' content knowledge, teachers' instructional practices, students' content knowledge, and students' practices in the context of seventh-grade English language arts. There were statistically significant effects on teachers' vocabulary and overall English language arts content knowledge and on vocabulary and writing practices. The effect sizes of these changes ranged from small to medium. There were statistically significant effects on students' reading comprehension practices. (Contains 5 tables and 4 footnotes.)