This handbook synthesizes accumulated research evidence about the main areas of language teacher education. It systematically applies research synthesis to the field, providing coherent, systematic insights into various aspects of language teacher education. Each chapter compares research conducted between 2010-2020 within a specialized area of teacher education. The chapters discuss the theoretical and research underpinnings of each area, describing the purposes, methods, and findings of the research, including the impacts of teacher education on teacher professional development and teaching effectiveness. The twenty-six chapters in this handbook address three main areas of teacher education: Teacher Variables, Teacher Professional Development, and Teacher Instructional Beliefs and Practices. Section One on teacher variables includes ten chapters focused on teacher reflective practice, identity, cognition, self-efficacy beliefs, emotion, motivation, demotivation and burnout, agency, autonomy, and nativeness/nonnativeness. Second Two includes five chapters on teacher professional development, namely good language teachers, preservice teacher mentoring, practicum in language teacher education, online language teacher education, and language teacher action research. Section Three consists of eleven chapters on teacher instructional beliefs and practices, which systematically review research on teacher beliefs and practices about English as an international language, teacher intercultural knowledge and beliefs, teacher curricular knowledge, TPACK of in-service teachers in language education, CLIL language teacher education, EMI language teacher education, heritage language teacher education, translanguaging in language teacher education, language teacher classroom discourse and interaction, language assessment literacy for teachers, and scaffolding and language teachers. This handbook is an invaluable resource for teacher educators, student/preservice teachers, inservice teachers, graduate students of Teaching English to Speakers of Other Languages (TESOL) and Applied Linguistics, and teacher education researchers.