The purpose of this study was to examine third-grade teachers’ support for students’ vocabulary learning in high poverty schools characterized by underachievement in reading. We examined the prevalence and nature of discourse actions teachers used to support vocabulary learning in different literacy lessons (e.g., phonics); these actions varied in the cognitive demands placed on the students. Results showed that teachers rarely engaged students in cognitively challenging work on word meanings. Various lesson features and student and teacher characteristics were associated with teachers’ support for students’ vocabulary learning (e.g., teachers’ knowledge about reading). A major finding was that the extent of teachers’ support of their students’ vocabulary learning was significantly related to gains in reading comprehension across the year.