This chapter describes a process for engaging middle school students in writing scientific explanations in the context of project-based science. In four Detroit classrooms, teachers enacted strategies for explanation writing that aimed to help students develop deep understanding of science concepts, inquiry processes, and writing skills. These four teachers and the university research team worked together to unpack the meaning of "explanation" as described in the National Science Education Standards (NSES) and to create a plan for helping seventh graders to understand that meaning. This chapter describes the resulting schema and the practices in which students engage in a chemistry unit on air quality and in a biology unit on communicable diseases. The chapter also describes the multiple ways in which to scaffold explanation writing across time so as to provide opportunities for all students to be successful using this form of scientific discourse.