The transfer of toxic chemicals to breast milk represents an important, although not widely recognized, chemical exposure route for the infant. For an increasing number of nursing mothers who resume their professional activities after giving birth, the obvious benefits of breast-feeding must be evaluated versus the risk of the lactational transfer of occupational chemicals to the infant. In this article, we review qualitative and quantitative data on occupational chemicals that may contaminate the breast milk of lactating women in the work force, and we discuss the possible use of physiologically based pharmacokinetic models to aid in the assessment of risk for infants whose mothers are occupationally exposed to chemicals.