The research article in this article is part of a larger study that examines an initiative to expand teacher expertise in facilitating mathematical problem solving within the framework of developing and field-testing pedagogical resources. We focus on one year of the study and report on the complex process of professional development as teachers move from traditional pedagogies of teacher explanation of mathematical operations followed by student practice to a pedagogy of teacher and student exploration of number operations within a problem-solving environment. We see that, in this initiative, teachers, consultants, and professional developers explored new mathematical and pedagogical ideas, creating a collective that provided a supportive environment that served as a model for classroom settings where teachers and students, in turn, investigated mathematical ideas. (Contains 1 figure and 1 footnote.)