Materials use is a critical yet understudied domain of language teaching across jurisdictions and educational contexts. This study explored what knowledge constituents that English-as-a-foreign-language (EFL) teachers mobilize in their use of materials. Based on the social theory of human cognition in tool use and the conception of foreign language teachers’ pedagogical content knowledge, a qualitative multi-case study involving eight EFL teachers at one university in China was conducted. Data from interviews, lesson observations, and documents across three consecutive semesters revealed that teacher knowledge for materials use is multi-dimensional and developmental. The constituents of this professional knowledge are manifested in four domains, i.e., subject matter, pedagogical, curricular, and contextual domains. The study concluded that the professional knowledge needed for materials use is not merely an individual trait, but mediated by both human and nonhuman elements. Therefore, new forms of agency from both teachers and materials are required to foster the growth of this professional knowledge. Based on the findings, suggestions were made for teacher learning through materials use in language classrooms.