The idea of a material constitution has become influential for at least two reasons. The first reason is the absence of coincidence between the scope of the rules of the formal or written constitution and the wider field of constitutional rules. Second, the idea of a material constitution also comes into play as some authors will define the constitution by a specific content or ’matter’. This chapters aims at clarifying the uses of the reference to a constitutional matter by exploring the form versus matter distinction. The core of our case can be summed up as follows: the form of the constitution is law; the matter of the constitution is politics. Politics, as a social activity, influences law as much as law, in turn, can govern political action to a certain degree. In this process, legal substance is as relevant as legal form. What matters, thus, is a relative degree of fitness between political activity (or matter) and law. A (sufficiently) ’good’ constitution allows for political activity to take place, while shaping it in keeping with basic constitutionalist values and principles. Such a constitution can be defined as a principled instrument of self-government.