There is continuing emphasis by many professionals and organizations on the importance of breastfeeding as optimal infant nutrition. Pediatricians are frequently asked about the safety of medications taken by the nursing mother and the risk to the infant. Most drugs and many chemicals will be transferred into milk. For a vast majority of these compounds, there is no risk to the infant. It is almost always possible for the mother to continue nursing while taking the necessary medication. This article presents an introduction to the pharmacology of the transfer of drugs into milk, discusses the importance of the infant's age in assessing safety and presents a number of maternal conditions for which drugs need to be used.