There are periods during perinatal development in which specific nutrients are required for optimal development, and there is growing evidence that optimal dietary intake of these nutrients, which include iodine, docosahexaenoic acid, choline, and folate, is important. Lessons in how these nutrient effects were identified can help us to broaden our approaches for finding other critical nutrients: we are looking for nutrients for which there is a wide range of dietary intake, that have no or marginal pathways for biosynthesis, and that are needed by dividing progenitor cells. For some of the nutrients discussed, such as iodine and folate, the effects in humans are abundantly clear; for others, animal data are the most convincing. More human studies need to be conducted. We need a better understanding of diet and diet supplement intake during pregnancy and lactation and of whether diets are particularly low in some nutrients. Also, we need to understand how common genetic variations influence nutrient requirements during these periods. If we are going to supplement maternal and infant diets, first we must understand much more about the risks of having too much of a critical nutrient. Whatever the limitations of our current state of knowledge, it is apparent that pregnancy and lactation are periods during which good nutrition is exceptionally important. The infant is not protected from the inadequate diet of the mother.