Teacher practices are essential for supporting students in scientific inquiry practices, such as the construction of scientific explanations. In this study, we examine what instructional practices teachers engage in when they introduce scientific explanation and whether these practices influence students' ability to construct scientific explanations during a middle school chemistry unit. Thirteen teachers enacted a project-based chemistry unit, "How can I make new stuff from old stuff?", with 1197 seventh grade students. We videotaped each teacher's enactment of the focal lesson on scientific explanation and then coded the videotape for four different instructional practices: modeling scientific explanation, making the rationale of scientific explanation explicit, defining scientific explanation, and connecting scientific explanation to everyday explanation. Our results suggest that when teachers introduce scientific explanation, they vary in the practices they engage in as well as the quality of their use of these practices. We also found that teachers' use of instructional practices can influence student learning of scientific explanation and that the effect of these instructional practices depends on the context in terms of what other instructional practices the teacher uses. (Contains 10 tables, 2 figures and 6 notes.)