Improvement of teachers' content knowledge has long been regarded as a priority in Indonesian school systems, and has been the goal for many teacher professional development programs. In this paper we report the evaluation of a professional development program to improve content knowledge for 147 Indonesian teachers. The teachers were divided into three experimental groups, and each group was treated to one of the three professional development models frequently used in developing countries. The results showed that all three models led to improvement in the teachers' content knowledge; however, the teacher-based scaffolding model proved to be the most effective. In our account of why this program of teacher-based scaffolding was successful, we compared it with other studies.