The aim of the present study was to determine the nature of the knowledge and skills related to professional expertise, and to develop means by which such expertise may be individually measured. An instrument was developed consisting of ® ve different scales (with contents ranging from 12 to 19 items each), each re¯ecting one of ® ve dimensions: knowledge, meta-cognitive knowledge, skills, social recognition, growth and ¯exibility. Participants were two different, but relevant, groups of raters: individual employees (N 5 558) and their immediate supervisors (N 5 454). The item-sets for both groups are nominally identical, except that the wording was adapted when necessary to suit one group or the other. The reliabilities of the developed scales were high with Cronbach's a ranging from 0.83 to 0.94, for both the self-ratings and the supervisor ratings. All ® ve scales are homogeneous and distinctive as corroborated by Multitrait-Multimethod and LISREL analyses. Background and construction of the instrument are discussed at length and the instrument is appended in two versions: the original Dutch questionnaire and an English version.