This study investigates the internationalization of internal audit functions by examining possible drivers of permanently employing internal auditors in foreign countries. As corporate scandals, the ongoing globalization, and increased environmental complexity necessitate new approaches to control, the internal audit function is faced with finding the most suitable organizational form for providing enterprise-wide assurance. After 14 in depth interviews with internal audit practitioners survey data from 295 chief audit executives from Germany, Austria, and Switzerland is used to construct an exploratory model of internal audit function internationalization. Positive effects on the degree of the internal audit function’s internationalization are observed for company complexity, audit committee and supervisory board oversight, the use of local expertise, firm size, and a company’s being in the financial sector. A focus on the internal audit function’s cost efficiency is negatively associated with its degree of internationalization. This paper adds to the growing literature on internal auditing while filling a research gap on internal audit function staffing practices in the international context.