This article examines the key change processes (KCPs) in the critical systemic functional linguistics praxis (CSFLP) of a science teacher, the first author, as she learned SFL theory as a metalanguage for thinking about language and for teaching science writing to culturally and linguistically diverse students. In this case study (Yin, 2012), which emerged from a large-scale action research project, we describe how the science teacher applied her developing knowledge of SFL to teach science writing over the course of five years. We analyze these changes through what we describe as three KCPs: (1) Changing conceptions of science writing; (2) Implementing the teaching learning cycle (TLC) to teach language and content; (3) Building teacher language awareness. Through the examination of these KCPs, we highlight the benefits of the TLC for learning to teach the genres of science writing over time (Rothery, 1996). Examples from the science teacher's classroom show how her implementation of SFL-informed instruction helped her apply the theory to scaffold the teaching of writing. Implications include discussion of how praxis occurs when teachers learn SFL theory and apply it to developing genre-based pedagogy in science.