Instructional scaffolding--the use of resources, structures, and verbal interaction to help students do challenging academic work--is known to be essential for effective English language reading instruction for emergent bilinguals (EBs; Goldenberg, 2010; Walqui, 2006). Yet little systematic research examines the scaffolds that teachers use. Drawing on scholarship about teacher cognition (Ben-Peretz, 2011) and instructional scaffolding (van Lier, 1988), this study analyzes how six middle school teachers, all nominated as exemplary, chose and used scaffolds during English language reading instruction. Data include video recordings of 60 reading lessons and 18 interviews. The researcher analyzed which scaffolds teachers used and how they explained their choices. As EBs read and discussed texts, teachers used a wide range of planned scaffolds (materials, structures, and routines) and interactional scaffolds (in-the-moment supports for language and content). Drawing on both formal and practical knowledge (Elbaz, 1991), they chose scaffolds that effectively provided comprehensible input, opportunities for language production and interaction, or support for reading development. The instructional context--class composition, available materials and equipment, and professional support--mediated teachers' choice and use of scaffolds. Findings inform teacher educators, school leaders, and researchers who seek to improve the scaffolding practice of preservice and in-service teachers working with EBs.