Breast milk is the most suitable nourishment for a newborn. After birth, the mammary glands produce colostrum, which is rich in proteins (including immunoglobulins). At later stages of lactation, the breast milk matures and changes its composition. Such changes are crucial to provide a young child with optimal conditions for proper development. Human milk contains lipids, proteins and carbohydrates as well as vitamins and minerals. Lipids are involved in the development of the entire nervous system, while proteins are essential for normal gastrointestinal and immunological function. Carbohydrates are responsible for the growth of intestinal microbiota and ensure resistance to microorganisms. Paediatricians recommend breastfeeding until the child is one year old, but this is not always possible. There are some conditions, concerning both mother and her child, for which breastfeeding is contraindicated. Such mothers may decide to use milk replacers based on cow’s milk. However, even the best milk replacer will never be as perfect for an infant as breast milk due to composition differences. This article summarises the current knowledge about the composition of breast milk and the role of its individual components as well as compares the properties of human and cow’s milk.