Action research, participatory research, and participatory action research methodology adopting critical theory as epistemology for knowledge development have been applied in diverse nursing specialties. They share similar research principles and values, and employ the same data collection methods. For these many scholars use the terms interchangeably. However, their developmental origins and ideological backgrounds are different, as are their expected outcomes and research methods. The terms have therefore been misused. This paper is intended to enhance researchers' ability to distinguish between the similar methodologies and to apply research methodology and related terms appropriately to the relevant issues of health care.