Adapting support contingently to student needs by first diagnosing their current understanding, that is, scaffolding, is considered a key aspect of excellent teaching. The use of classroom scaffolding is rare, however. We therefore investigated the benefits to teachers of a professional development program that was based upon a model of contingent teaching (MCT) with the following 4 steps: diagnostic strategies, checking of diagnoses, giving contingent support, and checking of student learning. In our experimental study, 17 of 30 teachers participated in this program. All of the teachers (prevocational education; teaching social studies) taught the same 5-lesson project on the European Union. The frequency and quality of their use of the 4 steps from the MCT were then compared. The teachers who worked with the MCT increased their teaching quality more than the teachers who did not participate, especially with regard to the steps of contingent teaching. They also showed more complete cycles of contingent teaching at postmeasurement than the other teachers. Less successful teachers showed a tendency to provide less support because they mistakenly thought that prompting was not part of scaffolding. Future scaffolding research and professional development efforts aimed at promoting scaffolding can benefit from the MCT, provided that teachers' understanding of scaffolding is closely monitored.