This article presents the results of an international comparative study of a widely neglected element in social health insurance: supervision upon the sickness funds as implementing agents of social health insurance. The following countries were included: Belgium, Germany, Switzerland and the Netherlands. A comparative analysis of the institutional structure of supervision revealed many differences. The goals of supervision are more or less the same in each country: preserving the lawfulness of implementation; ensuring trust and stability; preserving efficiency and supporting policymaking. The analysis of the supervision process focused upon three sub-processes: the collection of information; the assessment of the performance of the sickness funds and interventions to correct deviant behaviour. Finally, the analysis deals with changes in supervision, in particularly the impact of market competition in social health insurance upon supervision. It is argued that market competition will substantially alter the role of supervisory agents in social health insurance.