Integrated language and Science and Technology (S&T) instruction can make learning more contextualized and improve student learning outcomes for both subjects. Although it has been stressed in the literature that integrated language and S&T instruction presents pedagogical challenges for teachers, and therefore requires professional development, little is yet known about the specific characteristics of the required pedagogical teacher repertoire. To gain a better understanding of the teacher knowledge and skills required for integrated language and S&T instruction, a cognitive task analysis was conducted. Lesson observations and elicitation interviews with expert language and S&T teachers, and meetings with those teachers and subject matter experts, revealed teachers’ implicit and explicit knowledge and skills. This analysis resulted in a skills hierarchy consisting of various constituent teacher skills. Additionally, the cognitive task analysis revealed which knowledge teachers rely on and identified factors influencing the complexity of integrated language and S&T instruction. The findings presented here can serve as a starting point for the development of substantiated task-oriented professional development trajectories. Furthermore, it provides valuable insight into a relatively new and systematic way of researching teacher expertise.