Teacher action research seeks to bring together action, reflection, theory, and practice; and it is acknowledged as a way to value and honour teachers' practical knowledge. The purpose of this article is to conceptualize teacher action research as "Bildung," applying Gadamer's philosophical hermeneutics as a theoretical framework. Based on five teachers' lived experiences of conducting action research in their classrooms, this study explores how teachers go through the process of the development of the self as a teacher, furthering themselves as professionals through action research. It aims at understanding the role of action research as "Bildung," one's inner process of formation and cultivation of self, to shed light on teacher professional development as a hermeneutical and ontological practice. (Contains 1 table.)