This special issue recognizes EAPRIL as being a platform for practitioner and practice-based research and by organizing the 10th annual conference for practitioner research on improving learning in education and professional practice. Papers in this conference and in this special issue are rooted in practice-based research or practitioner research. They reflect the popularity of practitioner research in vocational teacher education and in universities of applied sciences. Reason enough for the authors of the current paper to reflect on the question: "What is practitioner research?" And, more importantly what makes good practitioner research? Reviews show that people use broad interpretations of the concept, which requires that to clarify the epistemological basis of the relation between research and practice the reflections goes back to Aristotle philosophy. The latter, aiming at discovering what kind of knowledge is obtained, what purpose it serve, and how it differs. This yields a theoretical, and two practical kinds of knowledge. Although all three are relevant, the so called "practitioner knowledge" (the prhonesis and the techne), need more attention in judging the "goodness" of practitioner research. Five principles of validation are mentioned, e.g. the process of meaning making and negotiation, differing from the correspondence between knowledge and the outside world, e.g. validity. These principles provide a possible angle and sometimes researcher follow them implicitly and unconsciously. The articles in this special issue reflects the realisation of many of these principles within the individual studies. The current paper does not intent to give final answers, but rather to trigger a further conversation on the fundamental question of: What is good practitioner research?