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 teaching.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 impacts of teacher education on teacher gains and teaching effectiveness. Areas addressed in this handbook include: teacher identity, motivation, demotivation and burnout, reflective practice, action research, Content and Language Integrated Learning (CLIL) teacher education, English Medium Instruction (EMI) teacher education, self-efficacy, assessment literacy, language awareness, Technological Pedagogical Content Knowledge (TPACK), supervision and mentoring, and nativeness/non-nativeness. 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.