The known facts about nucleation phenomena in liquid metals are interpreted satisfactorily on the basis of the critical size and interfacial energy concepts. In large continuous masses nucleation is almost always catalyzed by extraneous interfaces. However, in very small droplets the probability that a catalytic inclusion is present is so much less that their minimum nucleation frequencies are reproducible and form a consistent set of values.Interfacial energies, &sgr;, between crystal nuclei and the corresponding liquids have been calculated from nucleation frequencies of small droplets on the basis of the theory of homogeneous nucleation. Energies of interfaces, &sgr;g, one atom thick and containingNatoms were calculated from the &sgr;'s. The ratio of &sgr;gto the gram atomic heat of fusion, &Dgr;Hf, was approximately 0.45 for most metals but ∼0.32 for H2O, Bi, Sb, and Ge.The effect of relative complexity of crystal structure upon the supercooling behavior of pure metals apparently is a reflection of its effect upon &Dgr;Hf.