We discuss an original study and two replications conducted over 18 years examining the impact of Teacher Study Group (TSG) in Vocabulary, a professional development (PD) approach for improving pedagogy and student learning. Study 1 focused on the design and development of the PD and included a small-scale randomized controlled trial to assess the efficacy of the TSG model with grade 1 teachers. Study 2 was a scale-up study conducted in grade 1. Study 3 examined the efficacy of the TSG when implemented with grade 8 teachers of American history. We highlight similarities and differences in methodology and results among the studies and conclude with a discussion of the pattern of findings followed by a reflection on issues emanating from this line of research.