This longitudinal study examines the effects of a collaborative evaluation process on mathematics instruction and student outcomes in an elementary school serving a low-resource community. Thirty-two elementary teachers participated in a 3-year collaborative evaluation professional development process that contributed to improved mathematics teaching and student learning. Through critical-incidents analysis of qualitative data and through comparisons of means from quantitative survey data per analysis of variance, changes in teachers' practices were found after participation in long-term inquiry. The collaborative evaluation professional development process is described. Through this collaborative evaluation approach, teachers were able to make data-based decisions, take a leadership role in their daily work, and move toward using evidence to develop shared values and norms. They in turn used the evidence to making teaching and learning more deliberate.