In the Netherlands, an innovation is taking place in the science subjects in the upper levels of secondary schools aiming at more appreciation of the sciences and at better learning results of the students. New teaching materials following the so called concept-in-context (CoCo) pedagogical approach are being created in design teams comprising teachers. These teachers are expected to professionalise towards CoCo proficiency as a result of their participation in the teams. Apart from CoCo teaching competencies, CoCo proficiency comprises competencies in designing CoCo material and in helping implement CoCo in school. A key question addressed in my PhD project is: how does the participation of teachers in CoCo design teams contribute to their professional development towards CoCo proficiency, and which factors concerning the teams hinder or facilitate this. It is addressed in a multiple case study (n=32) to evaluate the CoCo proficiency of teacherdesigners (n=24) from various teams. Their CoCo proficiency will be compared both mutually and with non-designing teachers (n=8) and attributed to design team characteristics. This round table is especially concerned with the construction and validation of an instrument for measuring CoCo proficiency and its’ aspects and the description of the design teams. This project is part of the DUDOC-research program giving teachers the opportunity to perform a PhD research project.