This edited book is about preparing pre-service and in-service teachers to teach secondary-level mathematics to English Language Learners (ELLs) in twenty-first century classrooms. Chapter topics are grounded in both research and practice, addressing a range of timely topics including the current state of ELL education in the secondary mathematics classroom, approaches to leveraging the talents and strengths of bilingual students in heterogeneous classrooms, best practices in teaching mathematics to multilingual students, and ways to infuse the secondary mathematics teacher preparation curriculum with ELL pedagogy. This book will appeal to all teachers of ELLs, teacher educators and researchers of language acquisition more broadly. This volume is part of a set of four edited books focused on teaching the key content areas to English language learners. The other books in the set focus on teaching History and Social Studies, English Language Arts, and Science to ELLs.