BACKGROUND: Mother's milk is the recommended source of nutrition for all newborns. Preterm infants may be further compromised by maternal factors that impede successful lactation and alter milk composition. PURPOSE: To review and summarize the state of the science regarding implications of maternal overweight and obesity on successful lactation and associated alterations in preterm mother's milk composition. METHODS/SEARCH STRATEGY: PubMed, EMBASE, and Web of Science searches were performed using relevant key words to identify references addressing maternal overweight or obesity, prematurity, human milk, and lactation. FINDINGS/RESULTS: In the United States, more than half of women enter pregnancy with an overweight or obese body mass index. These women have increased risk of adverse pregnancy outcomes and obstetric complications that can undermine successful initiation and continuation of lactation, including preterm birth. Maternal overweight and obesity are also associated with alterations in mother's milk composition. IMPLICATIONS FOR PRACTICE: Mother-preterm infant dyads affected by maternal overweight and obesity are at risk for barriers to initiation and continuation of lactation. Support for early initiation of milk expression is needed. Continued support, especially during the first weeks of lactation, can facilitate sustained milk production. IMPLICATIONS FOR RESEARCH: Considerable knowledge gaps remain in this area of human milk science. Future research is needed to facilitate more comprehensive understanding of differences in milk composition associated with maternal overweight and obesity and their impact on clinical outcomes in the preterm infant.