Hierarchy is a type of systemic organisation into levels that are ordered with reference to criteria of a normative character, and fully or partially subordinated by relationships of power, influence, or control. Many intermediate situations are observed between strictly designed hierarchies where levels are distinct and communication restricted to vertical top-down command, and more flexible structures where the levels are not easily recognisable and the network circulation of information reveals unequal degrees of accessibility or control only after a detailed analysis. We are interested in hierarchies because they are at the core of many complex systems.