The modern constitution is predominantly understood as a way of instituting and limiting power, and is expected to contribute to (societal) stability, certainty, and order. Constitutions are hence of clear sociological interest, but until recently they have received little sociological attention. I argue that this is unfortunate, as a sociological approach has much to offer in terms of a complex and historically sensitive understanding of constitutions and constitutionalism. Constitutional sociology has particular relevance in contemporary times, in which the meaning of constitutions and constitutionalism is uncertain, and subject to contestation, and possible transformation. The constitutional sociology developed here is phenomenologically inspired and stresses the importance of understandings of the modern constitution as ‘embedded’ in constitutional imaginaries. Rather than as a visible and rationally designed construct, constitutional sociology understands constitutionalism as ultimately a ‘field of knowledge’. The suggestion is that this field of knowledge or ‘modern constitutional horizon’ is characterized by a tension between two ultimate markers, in terms of what Castoriadis has identified as the social imaginary significations of mastery and autonomy. Mastery and autonomy form prominent constitutional orientations, historically taking the form of solidified, instituted meanings, identified here as the modernist and the democratic imaginaries. In the last section, the two instituted constitutional imaginaries will be ‘unpacked’ in specific components (state sovereignty, absoluteness, fabrication, endurance, and distrust regarding the modernist imaginary; indeterminacy, creativity, dynamism, self-government and popular sovereignty regarding the democratic one). In conclusion, I suggest that constitutional sociology might significantly help elucidating the potential losses and heteronomous tendencies that may result from the contemporary uncertainty and possible metamorphosis that affects the modern constitution.