Planned instructional scaffolding--the temporary use of materials, routines and structures to support students in doing challenging academic work--is a critical component of English-language reading instruction with emergent bilinguals (students formally categorized as English Language Learners). Although scholars describe how scaffolds "should" be used to support language and literacy development in classes, no extant research describes how teachers actually choose and use a range of scaffolds that are responsive to the varied skills and needs of larger, diverse groups of emergent bilinguals, all within the complex and dynamic environment of a classroom. Drawing on Walqui's conceptual framework for use of scaffolds with emergent bilinguals and Corno's theories of adaptive teaching, this comparative case study of three exemplary teachers' reading instruction in middle school sheltered English classes focuses on how teachers selected and used instructional scaffolding "contingently," by adapting the support to the needs of an individual or the group. Following the call by van de Pol et al., this research also investigates how the teachers' use of scaffolds was shaped by their immediate context. Data, which include 30 video-recorded reading lessons and 9 teacher interviews conducted over one school year, were analyzed using qualitative methods. Findings show that the teachers' use of a range of whole-class and small-group planned scaffolds was shaped by their understandings of their class context, namely the heterogeneity of their classes and their adolescent students' socioemotional needs. The study's implications for teacher educators, coaches and researchers are discussed.