Science teachers are increasingly expected to use inquiry based science teaching [IBST] methods. According to the S-TEAM and ASSIST-ME European projects, IBST methods lead students to understand key scientific ideas that help them to make sense of the phenomena in the world around. Students are supported in using process and skills employed by scientists: questioning, reasoning, searching for relevant documents, observing, conjecturing, data gathering and interpreting, investigative practical work and collaborative discussions, and working with problems from and applicable to real-life contexts. This is a complex combination of objectives. This complexity claims for a better understanding of the set of teacher professional knowledge that is required by IBST methods. This presentation addresses the ways for identifying this type of knowledge and for understanding its development.