Young children in many US classrooms have limited opportunities to learn mathematics because their teachers are not well-prepared. The TRIAD model’s professional development has shown large significant effects on early childhood teachers’ beliefs and practice, as well as these teachers’ positive effects on children from low-resource communities, especially children with minority racial backgrounds. At the core of TRIAD are research-validated learning trajectories (LTs). We describe the professional development (PD) strategies we used, including an online tool, the “Learning and Teaching with Learning Trajectories” application (LearningTrajectories.org) that was central to the PD. We discuss the importance of LTs, then TRIAD’s use of LTs as the core of the approach to PD, and end with the empirical evidence supporting the efficacy and equity of this approach in mathematics teacher education.