Self-assembled structures or, as they are otherwise called, association colloids are spontaneously formed supramolecular entities. In most cases, the constituting molecules are amphiphilic, that is, they contain a lyophobic and a lyophilic part. In a given solvent and beyond a critical concentration, the amphiphilic molecules spontaneously organize themselves to give structures of colloidal dimensions. Supramolecular assemblies may also be spontaneously formed because of electrostatic interaction between oppositely charged compounds, for example, polycations and polyanions. This phenomenon is referred to as co-assembly, which is out of the scope of this chapter.