Through non-covalent or covalent assembly of carbon dots (CDs), functional CD assemblies (supra-CDs) with versatile architectures can be created. Among non-covalent forces, various supramolecular interactions can be used for this purpose, including amphiphilic interactions, H-bonding, electrostatic and π–π interactions, and van der Waals forces. Among covalent ones, chemical reactions including coordination interactions and interparticle/intermolecular covalent condensation of CDs result in the formation of supra-CDs. All these different kinds of interactions of CD building blocks have an impact on energy band structures and photophysical and photochemical processes and thus endow supra-CDs with intriguing optical properties. We review the recently emerged research field of supra-CDs in terms of their synthesis, morphologies, assembly-induced optical properties such as emission quenching or enhancement, occurrence of polarized emission and optical chirality, and underlying mechanisms. Their specific advantages for applications such as photothermal conversion and photodynamic therapy are presented, in order to inspire further development of supra-CDs.