The aim of this text is to introduce the essence and arguments for the usefulness of action research as a research paradigm in the development and updating of teachers' professionalism. Thus, I present the specific features of action research, its cognitive and practical potential in relation to everyday educational practice. I stress the importance of using researcherspractitioners own voices to express their concerns and opinions, and take new bold initiatives. Then I point out the cognitive interests which direct the action research on specific cognitive purposes and changes assumed. Next, I present the possibilities of applying this research approach in the process of academic education of teacher candidates as well as the process of professional improvement of teachers-practitioners. In both cases, I use a few samples of practices to illustrate my point of view on this paradigm. As an example of the initial teacher education I present an experience of E. Wołodźko from her book Towards an autonomy of study (2013). In second case, I mention experiences of my PhD students-teacherspractitioners.