This is a reflection on a case of in-service teacher education. Two Swiss teachers, assisted by a change agent, were developing an innovative teaching approach, inspired by Wiggins & McTighe's methodology Understanding by Design (UbD). While one developed a real understanding and mastery of this approach--improving therefore his professional skills--the other did not. The analysis developed through the following pages tries to better understand why this happened. The situation is examined through the scrutiny of the relationship between teachers' motivation and their understanding of UbD, using a mixed methods methodology.