© 2018 The Authors Our knowledge includes irreducible tacit elements which are related to the individual's personal nature that go beyond what we can express, which makes it very difficult to formalize, communicate and share. As this tacit knowledge consists of either actions or personal attitudes, we propose an approach to acquisition of tacit knowledge based on an ontological model. The ontology is built top down by changing the actors’ cognitive focus from the focal to the subsidiary, or from the aim of an action to its detailed objectives. We also use explicitation interviews and self-confrontation techniques to identify the tacit elements in actors’ activities, such as the concepts of situation, know-how and know-that, which constitute our ontology for knowledge acquisition.