Operating at arm’s length of the political process, independent agencies are exempted from the accountability mechanisms inherent in the ministerial hierarchy. Alternative accountability instruments are, therefore, often introduced in the design of these organizations. This thesis focuses on these instruments, and addresses three related questions: (1) What instruments of accountability are applied to independent agencies? (2) What variation in degrees of accountability can we observe? (3) Why are some independent agencies subject to more accountability instruments than others? Accountability arrangements are set up both by politicians and by independent agencies themselves. An analytical and empirical distinction is, therefore, made between mandatory accountability – imposed on organizations by politicians – and voluntary accountability – introduced by the organizations themselves. Furthermore, as the hypotheses on variation in accountability suggest differences across policy areas and political systems, the empirical study is divided into two parts. The first part compares a wide range of independent agencies in different policy areas within one country – the Netherlands; while the second part compares independent agencies operating in a single policy area, competition policy, across a wide range of different political systems. The study finds that independent agencies are subject to a variety of accountability instruments, and to varying degrees of accountability. Mandatory and voluntary accountability go hand in hand: organizations with high scores on the one also tend to have high scores on the other. Issue salience is a strong predictor of both types: agencies dealing with more salient issues are subject to more extensive accountability arrangements. Depoliticization plays a role in mandatory accountability, and when more veto players are involved in the policy-making process they tend to introduce fewer provisions for accountability. Other factors which matter for mandatory accountability are the salience of the issue of accountability itself, the level of trust in the civil service, the number of stakeholders of the agency, and the agency’s legal status and powers. Factors which matter for voluntary accountability are the democratic experience of, and stakeholders in, the polity, as well as the age, legal status, resources, and competences of the organization.