SummaryThe more recent animal studies reveal the maternal origin of many placental and fetal lipids. In most studies the degree of transport across the placenta and the degree of fetal or placental synthesis of lipids varies with gestational age. Apparently, earlier in pregnancy, there is more dependencé on maternal lipids to provide placental and fetal lipids. In most lipid classes studied the placenta has the capability of altering those lipids presented to it by selective transport and interconversions. Also, most lipid classes are synthesizedde novoin the placenta.This review of available information illustrates the many difficulties in describing placental lipid metabolism and transport. The first of these is the extrapolation of data from any other species to man. We hope that increased sophistication in the use of placental perfusion and in the preservation of human placental tissue will lead to the determination of the transport mechanisms in man as well as in other animals. Secondly, composition studies are of value only in that they point the direction in which metabolic studies should proceed. The third problem is that thein vitrometabolic studies, which tell us what reactions may occur in placental tissue, are not applicable to thein vivosituation, since the magnitude of these reactions is unknown. This information can come only from the perfusion of the placenta with the substrates in question. Finally, it appears that the correlation of placental lipids to maternal‐fetal disease is not going to be easily shown. Perhaps the best approach is the definition of the functional capacity of the placenta to perform metabolic alterations of certain lipids. Then any alteration in this functional capacity of the placenta might well be more meaningful than an alteration in the lipid pattern of the placenta in disease states. In all the studies to be performed, a great need exists for the determination of composition and metabolic pathways in relationship to the time of gestation. Studies should be done at different periods of gestation for both the metabolism and transport of lipids appears to vary with gestational a