The world of educational research is a strange one for a teacher to enter. This was very true for me as a biology teacher, having my origins in the "harder" sciences and trained to measure trees, not people. Naively, when I started my PhD study I expected to find a clear set of methods for investigating teacher knowledge and behavior. What a shock to find myself in this hazy environment full of conceptualizations and constructs, overlapping but disconnected, with names such as beliefs, intentions, cognitions, principles, orientations, and knowledge that could be formal, situated, personal, or practical. I was interested in knowing what teachers do when designing lessons, how this relates to their knowledge base, and how lesson design can be learned, which seem to be straightforward questions, but satisfying answers proved difficult to find. A study of the literature taught me that until the 1970s the main aim of studies on teacher planning was to describe teacher behavior and to assess whether teachers follow formal planning models. The conclusion was that they usually do not plan according to goal-oriented models (Yinger & Hendricks-Lee, 1994). From the 1970s onward, attention shifted to teachers' cognitions. Lesson planning was considered a process of either solving problems or of decision making, with teachers generating alternatives and choosing between them (Shavelson, 1973). Later, Shavelson himself criticized this perspective because it underestimated the role of routines and the influence of classroom materials (Clark & Dunn, 1991;Shavelson & Stern, 1981). More recently Maher Hashweh, who has played an important role in developing the concept of Pedagogical Content Knowledge, could not but observe that there is still limited knowledge on the interplay between teacher knowledge and educational design (Hashweh, 2005), that is to say, we lack answers to the questions about how teachers' knowledge develops during the design of lessons, and how this knowledge guides the design process.While searching the literature for answers, every once in a while I ran into quotations from the influential book The Reflective Practitioner by Donald Schön (1983) in which I recognized my own experiences as a teacher. Could Schön's ideas help me understand how teachers know