Sterols, or in rare cases structurally similar molecules, are biosynthesized or at least required by all eucaryotic organisms, as well as by many procaryotic ones, regardless of their status as plants, animals, or protista. This information, together with quantitative, structural, metabolic, and other data is reviewed. It is interpreted to mean that the primary role sterols play in nature is a nonmetabolic one as architectural components of membranes and that this role can be played, but less well, by other molecules which approximate the steroidal structure. The biosynthetic process should, therefore, and actually does not appear to be correlatable with this role, which, in turn, is correlatable with phylogenesis. The Δ24-reduction-alkylation bifurcation, for instance appears to be interrelated profoundly with the evolutionary differentiation of the animal from the plant kingdom.