This paper takes an explanatory approach to the triggers of Human Resource (HR) outsourcing decisions and evaluates them empirically. Our data show that many German firms have never explicitly considered outsourcing of HR functions. Obviously, explicit outsourcing decisions do not come into being automatically but have to be triggered. We analyze theoretically and empirically which triggers are relevant. In our definition, HR outsourcing includes a broad range of internal HR functions and the respective, externally procured personnel services (e. g., temporary agency work, payroll accounting, interim management, outplacement services, HR consulting, placement services and others).