Integrated language arts and science & technology (ILS&T) instruction can make learning more meaningful and improve student learning outcomes for both subjects. Although the literature has stressed that such integrated instruction presents pedagogical challenges for elementary school teachers, little is yet known about the specific characteristics of the required pedagogical repertoire. To gain a better understanding of the teacher knowledge and skills required for ILS&T instruction, a cognitive task analysis was conducted. This analysis resulted in a hierarchy consisting of the various required constituent teacher skills for ILS&T instruction. The cognitive task analysis also revealed what knowledge teachers rely on and identified the factors influencing the complexity of ILS&T instruction. The findings presented here contribute to our knowledge about what ILS&T instruction requires from teachers and can serve as a basis for the development of substantiated, 4C/ID-based teacher professional development trajectories. Furthermore, it provides valuable insight into the pros and cons of a relatively new, systematic way of analyzing teacher expertise.