The concept of spatial and electronic structure of cyclic molecular systems displaying the effects of cyclic electron @D01583@ which provide for their enhanced thermodynamic stability (relative to acyclic structural analogues) and tendency to retain the structural type in the course of chemical transformations. A quantitative assessment of the degree of aromaticity is given by the value of the @R05333@. It may also be evaluated by the energies of relevant isodesmic and homodesmotic reactions. Along with energetic criteria of aromaticity, important and complementary are also a structural criterion (the lesser the alternation of bond lengths in the rings, the greater is the aromaticity of the molecule) and a magnetic criterion (existence of the @D01668@ ring current induced in a conjugated cyclic molecule by an external magnetic field and manifested by an exaltation and @AT06776@ of @M03689@). Although originally introduced for characterization of peculiar properties of cyclic conjugated @H02889@ and their ions, the concept of aromaticity has been extended to their homoderivatives (see @H02839@), conjugated @H02798@ (heteroaromaticity), saturated cyclic compounds (σ-aromaticity) as well as to three-dimensional organic and @O04328@ (three-dimensional aromaticity). A common feature of the electronic structure inherent in all aromatic molecules is the close nature of their @V06588@ electron shells, i.e., double electron occupation of all bonding MOs with all antibonding and delocalized nonbonding MOs unfilled. The notion of aromaticity is applied also to transition states.