In this 21(superscript st) century, knowledge has exploded in volume and is disseminated through the Internet so fast that many concepts have been radically changed. In the academic world, new theories and concepts in various disciplines or fields have also been proposed to respond to the reality of the ever-changing world. This phenomenon is termed as ”paradigm shift” by Thomas S. Kuhn (1970). Under such conditions, this paper would like to revisit action research to review what it is and how it can be practiced by teachers in classrooms, especially language teachers in the context of the 21(superscript st)-century and in the particular context of Taiwan. The paper reviews the definition, origins, and research processes of action research. Then, it addresses how action research can be conducted by language teachers. The aim is to reemphasize the multiple roles of teachers as researchers as well as life-long learners. In sum, this paper pinpoints that knowledge creation should be produced through not only a top-down process but also a bottom-up process to create experience-based knowledge with a sense of ownership among the practitioners-the teaching professionals.