A wet nurse is a woman who breastfeeds and cares for another's child. Wet nurses are employed if the mother dies, or if she is unable or chooses not to nurse the child herself. Wet-nursed children may be known as "milk-siblings", and in some societies, the families are linked by a special relationship of milk kinship. Wet-nursing existed in societies around the world until the invention of reliable formula milk in the 20th century. The practice has made a small comeback in the 21st century.

Breastfeeding, or nursing, is the process by which human breast milk is fed to a child. Breast milk may be from the breast, or may be pumped and fed to the infant. The World Health Organization (WHO) recommends that breastfeeding begin within the first hour of a baby's life and continue as often and as much as the baby wants. Health organizations, including the WHO, recommend breastfeeding exclusively for six months. This means that no other foods or drinks, other than vitamin D, are typically given. The WHO recommends exclusive breastfeeding for the first 6 months of life, followed by continued breastfeeding with appropriate complementary foods for up to 2 years and beyond. Of the 135 million babies born every year, only 42% are breastfed within the first hour of life, only 38% of mothers practice exclusive breastfeeding during the first six months, and 58% of mothers continue breastfeeding up to the age of two years and beyond.