This article describes the lessons learned during the implementation of a new master's degree program for K-8 teachers. The program's goal was to increase student inquiry and sense making in mathematics and science classrooms. Each of the twenty-four teacher participants reported strong satisfaction with the program, and a variety of measures showed improvements in teachers' math and science content knowledge and instruction. Despite these successes, some difficulties were encountered, including anxiety among the teachers at particular points in the program, disconnects between the program's ideals and classroom realities, and problems linking changes in instruction with K-8 student outcomes. Key decisions and dilemmas underlying these difficulties are discussed, organized around the themes of program structure, course content/pedagogy, and evaluation.