The worldwide prevalence of childhood asthma has been increasing considerably, and the protection afforded by breastfeeding in its development has been the subject of controversy for more than 80 years. Previous systematic reviews have generally found a protective effect of breastfeeding on allergic outcomes, although many studies have methodological limitations. Although breastfeeding is protective against lower respiratory tract infection during infancy, such protection has not been demonstrated for asthma in all studies. Breastfeeding has health benefits for the mother and child. Exclusive breastfeeding for the first 6 months of an infant's life, with continued breastfeeding for up to 2 years or longer, is recognized as the "gold" standard for infant feeding because human milk is uniquely suited to the human infant, and its nutritional content and bioactivity promote a healthy development. There is increasing concern that the practice of delaying complementary foods until 6 months may exacerbate the risk of allergic disease. Breast milk contains immunological components that protect against infections and allergic disease in infancy. The composition of human breast milk is complex, containing factors that interact with the infant immune system and intestinal milieu including allergens, cytokines, immunoglobulins, polyunsaturated fatty acids, and chemokines. Transforming growth factor β is a cytokine in human milk involved in maintaining intestinal homeostasis, inflammation regulation, and oral tolerance development. Modern day society, with increased standards of hygiene, has changed the gut flora of Western infants, potentially impacting the risk of developing immune-mediated diseases including allergic disease and asthma. Microbial diversity is intrinsic to healthy immune maturation and function. Compared to breastfed infants, formula-fed infants had lower bacterial diversity and an altered intestinal microbiota in the first few weeks of life associated with an increased risk of eczema and asthma. Favorable gut colonization through continued breastfeeding may promote tolerance as well as protection when complementary feeding is initiated. © 2017 S. Karger AG, Basel.