To acknowledge the lasting and increasing influence of Halliday's theory of systemic functional linguistics (SFL) on teacher education, this article outlines Critical SFL Praxis (CSFLP) in teacher education as an activist effort in research and practice. Informed by systemic functional linguistics, the critical approach seeks to push back against minoritization of multilingual youth in schools and communities. Profiling the socio-historical background of SFL-based pedagogy in teacher education in Australia, the authors of this article conducted interviews with four SFL scholars and teacher educators from Australia who were greatly influenced by Michael Halliday and his theory of social semiotics. After a brief definition of CSFLP, we present the findings of our qualitative analysis of the interview with the four Australian SFL scholars. These findings illustrate the historical tensions, challenges, and opportunities in enacting CSFLP and point toward key affordances of the theory for teachers and teacher education.