One of the goals of education is to promote growth in reasoning; a critical subgoal is to promote competence in critically evaluating the quality of evidence (e.g., evaluating comparisons, soundness of measures, etc.). In a classroom experiment with 4 science teachers, 14 science classes, and 225 seventh graders, two methods of promoting growth in the ability to evaluate evidence were contrasted over 10 lessons in which students evaluated evidence to solve scientific problems. Half of the classes engaged in active reasoning—extensive group and class argumentation about evidence quality. The other half engaged in active reasoning plus expert modeling; these students engaged in the same argumentation but also listened to and discussed short conversational exchanges in which scientists discussed how they evaluated evidence quality. Students in the active reasoning plus modeling condition outperformed students in the active reasoning condition. Implications for the development of reasoning and for instruction are discussed.