Probably the oldest concept in the study of International Relations going back at least to the work of Thucydides. It is closely associated with both diplomatic parlance and realist theory. Its logic derives from the self‐help imperative of the international system's anarchic structure, in which states are obliged to give priority to survival and security. In pursuing this logic, states will usually join together to oppose any expansionist centre of power that threatens to dominate the system and thus threaten their sovereignty. Balance of power behaviour is central to conceptions of the national interest and to alliance policy. If successful, it preserves individual states and the anarchic structure of the system as a whole. Its opposite is ‘bandwagoning’, in which states seek security by joining with the dominant power. Realists conceive balance of power as an automatic tendency in state behaviour. In an international society perspective, balance of power is a conscious policy shared amongst a group of states, and serving as the principle by which they regulate their relations. Neither ‘balance’ nor ‘power’ are measurable, and their interpretation is much debated.