Aspects of the aromaticity concept are reviewed: aromatic characters and measures of aromaticity. Aromatic character is defined by certain physical and chemical properties that characterize cyclic organic molecules, including stability, reactivity, and ring current effects. Measures of aromaticity are numerical indices devised to understand and to predict aromaticity. Emphasis is put on recent developments of theoretical indices, especially the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) or chemical hardness.