Stephen Tierney’s new book presents an exciting new theory of federal constitutionalism. His core argument is that each federal constitution is founded on a ‘federal contract’, which is an agreement among a plurality of territorial communities to form a federal political community while retaining their distinct constitutional identity as constituent states. This offers an alternative to the ‘social contact’ of liberal political theory in which the isolated individual is the sole basic unit of the constitutional order. The explanatory power of Tierney’s thesis is impressive. He shows how the idea of the federal contract can be manifested in many varied forms but in a manner that recognises the constitutional status and autonomy of the constituent states, formed into an association under which they agree to act cooperatively with each other and with the federation as a whole, under a mutual commitment of reciprocity that underwrites inter-governmental cooperation and solidarity, while also maintaining inter-governmental distinctiveness and autonomy. Under the auspices of this simple and elegant idea, Tierney offers an account of federal constitutionalism that resembles the fractal patterns of mathematical geometry insofar as each federal system reflects a particular federating logic that drives its initial creation and ongoing evolution. This opens up a promising area of further research involving an ever-closer examination of federal systems to see whether the fractal-like patterns he has identified continue to be apparent at progressively higher levels of resolution.