This paper will examine the development of a group of Swiss teachers, through their assimilation of an innovative teaching approach, inspired by Wiggins & McTighe’s (1998) “Understanding by design”: an organized and constructivist teaching approach that can involve significant changes to normal practice. The project is part of a wider school improvement strategy within some Swiss vocational schools (Ostinelli, 2007, 2008), whereby teachers are supported by a School Improvement Advisor/researcher (SIA). The process was found to result in deepened understanding on the part of all three categories of actors (students, teachers and the SIA), with, effectively, each acting as reflective practitioners (Schön, 1984, 1987). It is therefore described as a circle of understanding.   Drawing upon data stemming from interviews, questionnaires and rubrics, Giorgio Ostinelli will examine this example of teacher development within the framework of Linda Evans’s (2008a,b) theoretical model of professional development.