The development of a taxonomy of human intellectual structure has received much impetus during the last decade. Findings indicate that there are at least 30-40 primary abilities, rather than only the seven proposed by Thurstone. While this list does not constitute a definitive taxonomy of primary ability structures, nevertheless, much of the ability variance should be accounted for by the presently known factors. At the second stratum level, several major factors (Gf, Gc, Gm, Gps, Gr, Gv, Ga) have been well replicated in numerous studies. Evidence indicates a smaller number of primary ability factors in children and old-aged individuals. Several of the ability factors have been shown to significantly predict educational achievement at both elementary and secondary school levels.