Within the context of managerial reforms in the Belgian public sector since the 1990, the internal audit function is viewed as a new managerial instrument to reinforce the reliability of an organization’s internal control system. It has spread sporadically amongst public entities. At the same time, recent research on internal auditing has shown that the professional boundaries of the internal audit continue to evolve, driven primarily by increased attention to “good governance†and the resulting regulations, codes of good governance, guidelines, and internal control standards. Bearing all this in mind, the main research question addressed here was to identify to what extent the institutional theory explained the choices of Belgian public entities to adopt IA programs and to professionalize them. The theoretical framework of this study argued that the institutional pressures are necessary but not sufficient conditions to explain IA outcomes. To complete the institutional framework of DiMaggio and Powell, an autonomy perspective of public entities has been suggested to explain IA administrative behaviour with regard to IA programs in the public context. The study examines the influence of the six conditions: the coercive pressure, the normative pressure, the managerial autonomy, the structural autonomy, the financial dependence and a strong external control environment. Their influence is examined on two phenomena: first on IA adoption; second on IA professionalization. IA professionalization is based on the maturity scale levels taken from the Internal Audit Capability Model (IIA, 2009); then simplified into three maturity states: unsustainable, sustainable and professional. Configurational hypotheses have been formulated combining both institutional pressures and autonomy attributes, assuming that various configurations of conditions lead to similar IA outcomes while some conditions are more powerful than others, depending on their interaction with each other. Consequently, this research design is not a ...