Typically, anarchy is considered the opposite of a constitutional order. This project complicates that picture. We reveal how anarchists constitutionalise, in other words, how and why they develop declarations, rules, institutions, and democratic decision-making procedures within anarchist groups to transform in the world in which they act. The research has a theoretical, empirical and practical/normative element. We examine the history of anarchist political thought to show how anarchists engaged critically with republicanism to detach constitutionalism from its conventional association with the state and private property. We explain how anarchists conceptualised ‘anarchy’ as an alternative to prevailing bourgeoise constitutions, and how they adopted constitutional principles of the division of power and federalism to resist tyranny. The second part uses this history to examine the constitutional practices of contemporary anarchistic movements: the US and UK Occupy Wall Street movement, the syndicalist union the Industrial Workers of the World (IWW), and a UK federation of anarchist cooperatives called Radical Routes. Using innovative coproduction methods, we investigate how effectively anarchists constitutionalise today. The third part utilizes critical insights from this learning process to describe anarchist constitutionalising practices that are adaptable to the needs of diverse community and horizontal grass roots movements, while also helping us rethink the meaning of and value of anarchy today.