This chapter presents a theoretically and empirically grounded approach to the why, what and how of deliberate attention to explanatory communication in a mathematics professional development (PD) project aimed at lower secondary-level teachers in South Africa. Called the Wits Maths Connect Secondary (WMCS) project, we focus on word use and justifications as specific language practices that work together to produce explanatory talk. Four principles that structure how we mediate word use and (mathematical) justifications in teachers’ productions in our PD practice are described and illustrated, with junior secondary algebra our content focus. These four principles and reflection on their enactment combine into an argument that content and context specificity matter in the enactment of language responsive mathematics teacher PD.