Science teacher educators (STEs) have key roles in the educational system through their preparation of pre- and in-service science teachers and in implementing future-oriented policy reforms. Given the complexity and importance of their role, what types of knowledge do STEs need? STEs come from a range of backgrounds and there are few systematic routes for their ongoing learning. Hence, there is a need for tools that can help define and assess STEs' competence. The purpose of this theoretical paper is to highlight central knowledge domains for STEs. We draw on previous work and add perspectives from current trends in science education and teacher education to describe four knowledge domains for STEs: natural science, science education in school, science teacher education and science education research. The main contribution of the paper is an updated conceptualisation of the nature of STEs' knowledge and qualifications, taking into account current requirements for future-oriented science education and teacher education including deeper learning and critical thinking skills, cross-curricular work, education for sustainable development, and research-based teacher education. The four knowledge domains can serve as a tool for identifying needs, recruiting staff with desired expertise, and tailoring interventions to support professional development for STEs from various backgrounds.