Online professional development (OPD) has great potential to improve teacher quality by improving teachers' knowledge and instructional practices, with the ultimate goal of improving student achievement. There is a dearth of scientific research, however, on the effects of OPD. This article presents the results of a randomized control trial exploring the effects of a series of three learning-community model OPD workshops on teachers' knowledge and instructional practices in the context of fourth grade English language arts. There were significant effects on changes in teachers' knowledge and instructional practices, as they related to the targeted goals in vocabulary, reading comprehension, and writing instruction. (Contains 1 figure, 11 tables and 3 footnotes.)