This book defends instrumentalism, the claim that political legitimacy rests on promoting justice. This entails that power can be permissibly exercised without consent, democracy, or public justification. In the book’s first part, a revisionary understanding of the problem of legitimacy is defended. Legitimacy is defined as the permissibility of exercising power. This entails that the problem of legitimacy is separate from the problem of justifying political obligations. Furthermore, legitimacy can exist without political authority. Instrumentalism is described and defended in the book’s second part. The view is based on a hybrid theory of justice that recognizes both aims and constraints. The importance of justice is defended against competitor views which prioritize stability or perfection. A series of suggestions is made how instrumentalism should be formulated, in particular, that it should take a sophisticated two-level structure. The third part of the book argues against three competing theories. It is argued that consent is not necessary for legitimacy, and that strong, moral rights can be made compatible with the instrumentalist view. Proceduralist views which ground legitimacy on democracy are also rejected, but the possibility that democracy is a ‘secondary value’ within instrumentalism is explored. Lastly, it is shown that legitimacy does not rely on justifiability to reasonable people, but that legitimate governments can nonetheless accommodate disagreement in natural ways.