The author describes interactions with two middle grades (grades 6-8, student ages 11-14 years) and three secondary school (grades 9-12, student ages 14-18 years) mathematics teachers designed to increase and enhance teachers' content knowledge and transform their classroom instruction by embedding the author (i.e., mathematics teacher educator) into teachers' practices. In addition, the author operationalizes the teacher educator-embedded professional learning model. Embedding a teacher educator into grade K-12 (student ages 5-18 years) teachers' practices, as presented in this study, involves more than simply implementing lessons with teachers. Rather, the mathematics teacher educator navigates iterative instructional cycles alongside the participating teacher, serving as sounding board, interventionist, epistemic student, and colleague. Results of teacher-educator embedding are presented, indicating participating teachers increased their content knowledge, engaged their students in more rigorous mathematics, demonstrated increased self-efficacy and more frequently engaged students in mathematical sense making, reasoning, modelling, generalizing, and communicating.