In this study we examined the effects of dimensions of teacher quality on students' comprehension and vocabulary performance. Participants were 36 teachers and their respective 679 students in 2 medium-size school districts in central Texas, both of which served high proportions of children from low-socioeconomic status households. We matched schools in Districts 1 and 2 on the previous year's reading achievement performance and then assigned them to 1 of 2 experimental conditions--comprehension or content vocabulary--through stratified random assignment, with each condition represented in multiple schools in each district. Teachers in each condition participated in a distributed 15 hr of content- and curriculum-based professional development over an 18-week period. The intervention was for 30 min, 3 times a week. We examined the following dimensions of teacher quality: teacher qualifications, instructional practices, quality of strategy use, treatment fidelity, and instructional effectiveness. We used student measures of reading comprehension, content vocabulary, and social studies knowledge to explore the effects of teacher quality. Teachers' education, fidelity, and indicators of teacher quality were significantly related to student outcomes on a standardized measure of reading comprehension. (Contains 1 figure and 7 tables.)