For some years, historical thinking and reasoning (HTR) have been important educational goals for upper secondary education in many countries. Nevertheless, teachers are often unsure of how to realise these ideas in the classroom. This article reports on the development of the domain-specific observation instrument Teach-HTR. It is intended for the further professional development of experienced history teachers who wish to foster historical thinking and reasoning, as well as to assist those who are doing their initial teacher training. The observation instrument was developed in several phases.