Human milk is the optimal source of nutrition for the nursing infant. Classically, the nutrients (water, protein, lipid, carbohydrate, vitamins, and minerals) were studied as the critical components of milk serving the growth needs of the infant for optimum growth. However, human milk contains factors other than the classically defined nutrients for which researchers are investigating potential roles in infant and maternal health, development, and well-being. The symposium addressed some of the exciting factors being studied, including microbes and maternal cells found within milk. Drs. Michelle McGuire and Juan M. Rodríguez addressed the presence of a bacterial community in human milk produced by healthy and mastitic mothers, potential sources of those bacteria, and the impact of milk-derived bacteria on the nursing infant. Drs. Donna Geddes, Peter Hartmann, and Foteini Hassiotou discussed the potential importance of maternal cells. For years, immune cells were known to be present in human milk, but recent evidence suggests that their impact is as much on the infant as on the health of the lactating mammary gland. Finally, the existence of highly plastic stem cells in human milk opens doors for previously unforeseen developmental “training” of the nursing infant.