The fetus is dependent upon the placenta for transport to it of all nutrients for energy and growth. The primary nutrients crossing the placenta are glucose, lactic acid, free amino acids, free fatty acids, and ketone bodies. Under normal circumstances, it appears that glucose is the primary energy source. In abnormal conditions there is the possibility that energy requirements may be met also by protein-amino acid oxidation and ketone body metabolism. The fetal brain may be quite adept in the use of ketone bodies. In all transport phenomena, the placenta intervenes by diverting nutrients into its own intermediary metabolism. For a more extensive consideration of these factors, several in-depth reviews are recommended. To better understand abnormalities of intrauterine growth, we need much more specific information about the mechanisms of placental transport and the intermediary metabolism of the placenta and fetus. When these are adequately in states of normal and abnormal intrauterine growth, we can then consider methods of metabolic intervention which will correct intrauterine growth failure and, hopefully, eliminate its hazards to the fetus and child.