Action research is a term used to describe a family of related approaches that integrate theory and action with a goal of addressing important organizational, community, and social issues together with those who experience them. It focuses on the creation of areas for collaborative learning and the design, enactment and evaluation of liberating actions through combining action and research, reflection and action in an ongoing cycle of cogenerative knowledge. While the roots of these methodologies go back to the 1940s, there has been a dramatic increase in research output and adoption in university curricula over the past decade. This is now an area of high popularity among academics and researchers from various fields - especially business and organization studies, education, health care, nursing, development studies, and social and community work. This encyclopedia brings together the many strands of action research and addresses the interplay between these disciplines by presenting a state-of-the-art overview and comprehensive breakdown of the key tenets and methods of action research as well as detailing the work of key theorists and contributors to action research.