This study investigated the instructional talk of three math teachers from two urban middle schools as they implemented a curriculum designed to develop students' knowledge of academic vocabulary and encourage word use during classroom discussions. Data were drawn from audio- and video-recordings, classroom observations, and teacher interviews. Results revealed two predominant differences among teachers' instruction that seemed consequential for each classroom's discussion-based learning environment. First, teachers' instructional foci and the information about words they emphasized, whether "definitional" or "contextual," were associated with differences in the kinds of talk generated about words and in the numbers of students who participated in discussions. Second, the ways in which teachers structured their discussions, as either "knowledge display" or "knowledge building," shaped students' interactions with the content and with each other, while also determining the kinds of opportunities students had for target word use. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]