This article gives an account of a professional procedure for the assessment of interpreters, namely the certification exams for social interpreters in Flanders, Belgium. After developing a professional profile for social interpreters, the authors align the profile of the graders with the certification exam. Special attention is given to standards, both for gate keeping of social interpreters and for the graders themselves. The introductory sociological framework underlines the importance of legitimization for institutions for gate keeping in the professional domain. Using the concrete test procedure as a starting point, the authors exemplify how competency based evaluation grids and expert knowledge of graders are determining factors for the legitimacy of the certifying institution.