Four criteria for aromaticity:

1. Cyclic
2. Planar
3. Completely conjugated (every atom has a p-orbital)
4. (4n+2) pi electrons (n = 0, 1, 2, ...)

Aromatic species: Benzene (6 pi e-), naphthalene (10), tropylium cation C7H7+ (6), cyclopentadienyl anion C5H5- (6), cyclopropenyl cation C3H3+ (2), pyridine (6), pyrrole (6 — lone pair of N is in the pi system).

Anti-aromatic: Planar, cyclic, conjugated but with 4n pi electrons. Examples: Cyclobutadiene (4 pi e-), cyclopentadienyl cation (4 pi e-). Anti-aromatic species are extremely unstable.

Non-aromatic: Not planar OR not fully conjugated. Cyclooctatetraene (8 pi e-) is non-aromatic because it adopts a tub shape to avoid antiaromaticity.