The components of the Kangaroo Mother Care (KMC) intervention, their rational bases, and their current uses in low-, middle-, and high-income countries are described. KMC was started in 1978 in Bogotá (Colombia) in response to overcrowding and insufficient resources in neonatal intensive care units associated with high morbidity and mortality among low-birthweight infants. The intervention consists of continuous skin-to-skin contact between the mother and the infant, exclusive breastfeeding, and early home discharge in the kangaroo position. In studies of the physiological effects of KMC, the results for most variables were within clinically acceptable ranges or the same as those for premature infants under other forms of care. Body temperature and weight gain are significantly increased, and a meta-analysis showed that the kangaroo position increases the uptake and duration of breastfeeding. Investigations of the behavioral effects of KMC show rapid quiescence. The psychosocial effects of KMC include reduced stress, enhancement of mother–infant bonding, and positive effects on the family environment and the infant's cognitive development. Conclusion: Past and current research has clarified some of the rational bases of KMC and has provided evidence for its effectiveness and safety, although more research is needed to clearly define the effectiveness of the various components of the intervention in different settings and for different therapeutic goals.