The objective of this review is to evaluate the evidence for a link between maternal obesity and poor lactation performance. In nonhuman species, excess maternal fatness is deleterious for lactation and also for maternal health and survival. These effects occur during pregnancy and as milk production is beginning. They may result in poor growth and survival of the young. In women, there is a negative association between maternal obesity and the initiation as well as the continuation of breastfeeding. This appears to be derived from biological as well as sociocultural factors that are still poorly understood. Excessive gestational weight gain, complications of pregnancy and delivery, and the condition of the infant at birth may also contribute to this association. Given the increasingly high rates of obesity among women of reproductive age worldwide and the importance of breastfeeding for infant health, further study of this association is essential.