The importance of probability increased in German teacher education in the last decades. However, research about the professional knowledge of (prospective) teachers focused on other content such as number theory and functions rather than probability. The project “The development of professional knowledge of prospective teachers in probability” aims at developing a test in order to measure teacher knowledge in probability. In order to measure content knowledge and pedagogical knowledge in this domain, one has to identify categories, concepts and ideas and connections between. This paper shows the development of a domain map, which indicates categories, concepts and ideas. The theoretical framework, the procedure of developing and an outlook is given.