For the best part of the last 35 years, action research has been expanding its influence as a research approach and source of professional development in the field of English language education and applied linguistics. While the number of research publications has grown considerably since the late 1980s when this movement began, there have been few systematic overviews of the type of research produced during this time. This chapter provides a synthesis of research on English language teacher action research conducted from 2010 to2020 in this field. The review aims to provide empirical evidence for the impact and implications of this body of research in language teacher education. It aims to integrate and interrogate what research has been carried out and the patterns of evidence or contradiction it has evinced. This research synthesis focuses on three interrelated areas: (i) studies illustrating the initiation and progression of action research in diverse cultural and institutional contexts; (ii) the impact of action research on the participants involved (e.g., teachers, facilitators, researchers) and their practice; and (iii) accounts of action research produced by teachers.