This article examines Finnish cooperating physics teachers' conceptions of teacher knowledge in physics. Six experienced teachers were interviewed. The data was analyzed to form categories concerning the basis of teacher knowledge, and the tradition of German Didaktik and Shulman's theory of teacher knowledge were used in order to understand the results. The results show that the teachers strongly emphasize knowledge related to day-to-day teaching practice. In German Didaktik, this means knowledge of the best instructional methods, while in Shulman's categorization it belongs to the category of representations, strategies, and methods. It was also found that the teachers appear to lack some of the essential key ideas of modern physics teacher education, such as a holistic view of instructional approaches.