The increasing attention in science education reform documents to language and literacy, coupled with the growing linguistic diversity of United States classrooms, means that science teacher educators and researchers are facing an important task of preparing secondary science preservice teachers (PSTs) to teach a language- and literacy-integrated form of science in linguistically diverse classrooms. Unfortunately, research in this field is in its nascency, and while some studies have addressed PSTs' instructional planning for and implementation of language- and literacy-integrated science instruction, none have considered the unique contextual factors that inform PSTs' uptake of these reform-oriented instructional practices, despite many studies suggesting student-teaching contexts are a key factor in PSTs' instructional decisions. The purpose of this multiple case study was to consider the unique contextual factors that supported and constrained three PSTs' uptake of language- and literacy-integrated science practices through a Cultural-Historical Activity Theory (CHAT) lens. Findings indicated that PSTs' implementation of language- and literacy-integrated science instruction was shaped by interactions among the mediating elements of their student teaching activity systems, including PSTs' division of labor with mentor teachers, the classroom and school norms, PSTs' perceptions of their classroom communities, and PSTs' past life experiences and resources. These findings highlight the roles that different elements of learning-to-teach contexts can play in PSTs' enactment of reform-oriented instruction. Implications for science teacher education, instruction for emergent bilinguals, and research are discussed.