Problems in the classification of -surplus heteroaromatic compounds and the criteria of their -donor character are examined in the review. The results of quantum-mechanical calculations, the ionization potentials, the tendency to form chargetransfer complexes with various electron-acceptors, the oxidation potentials, etc., are singled out as criteria of -donor character. It is shown that although high -donor character is a consequence of -surplus character, there is no quantitative relationship between these concepts, and they must be clearly distinguished.