It is well established that obese women are at increased risk of delayed lactogenesis and short breastfeeding duration, but the underlying causal contributors remain unclear. This review summarizes the literature examining the role of insulin in lactation outcomes. Maternal obesity is a strong risk factor for insulin resistance and prediabetes, but until recently a direct role for insulin in milk production had not been elucidated. Over the past 6 y, studies in both animal models and humans have shown insulin-sensitive gene expression to be dramatically upregulated specifically during the lactation cycle. Insulin is now considered to play a direct role in lactation, including essential roles in secretory differentiation, secretory activation, and mature milk production. At the same time, emerging clinical research suggests an important association between suboptimal glucose tolerance and lactation difficulty. To develop effective interventions to support lactation success in obese women further research is needed to identify how, when, and for whom maternal insulin secretion and sensitivity affect lactation ability.