Summary. The nature of expert knowledge is explored from two points of view. One of them questions the possibility of expert judgement making in practice and analyses the possible origins of bias of professional decision making. The second approach assumes that expertise manifests in different approaches in solving problems between beginners and experts who have proven high competence in the field. Shanteau’s analysis of particular fields of expertise which shows differences in validity and reliability of professional decision making was also described. These differences to large extent can be attrib- uted to different characteristics of problems specific to a particular field. In The last section of the paper gives an overview of the research results considering the origins of bias in medical expertise.