Abstract This paper addresses expertise as the foundation of professional boundaries and domains through a comparative study of four eye care occupations in the Netherlands. Claims of expertise are explored with an analysis of whether practitioners believe that expertise is exclusive to their profession. Results show that (a) established professions display a stronger sense of the ‘exclusiveness’ of expertise; (b) idiosyncratic expertise is more common among encroaching professions than among established ones. These findings substantiate trends towards professionalism, as jurisdictional disputes on professional domains and boundaries usually occur between more established medical professions seeking to protect their knowledge area, and encroaching ones trying to expand theirs. The study next addresses the ways in which claims to expertise influence practitioners’ attitudes towards professional status and professional practice. Attitudes to expertise are influenced by age, and affect professional work in the workplace. Practitioners who considered expertise as a collective characteristic were younger, undertook more examinations, more diagnoses, gave more treatments, referred less, and perceived more problems regarding inter‐professional recognition of their professional status.