Although breast-feeding has long been associated with lowered infant morbid- ity and mortality from infectious disease, until relatively recently little was known regarding the individual components of human milk aside from their nutritive func- tions and the presence of secretory antibodies. Over the last 40 years, and especially over the last decade, evidence has been growing that human milk contains a large number of materials that are bioactive and that are not found in artificially formu- latedinfantdiets. Disparatelinesofresearcharecurrentlyproducingsurprisinglylong listsofnewlyrecognizedhumanmilkcomponents-antimicrobialsand immunomod- ulators, includinganti-inflammatoryagents, antioxidants, cytokines, andhormones- with biological activities that relate to pathogenesis, inflammation, development, metabolic regulation, and other functions. The sum of all of these biologically active milk components may account for the strong protection that human milk affords nursing infants. Strictly speaking, most components of human milk could be considered bioac- tive, since nutrients are bioactive by definition. A major emphasis of this book, how- ever, is on defining what is known about components of human milk that inhibit common pathogens of the infant, those that have hormonal and/or cytokine activity, those that have immunomodulatory and/or anti-inflammatory activity, xenobiotics, and nutrients that are uniquely essential to early development. The topic of bioactive substances in human milk was explored in depth at the th 8 International Conference of the International Society for Research on Human Milk and Lactation (ISRHML) held at Plymouth, Massachusetts, October 25-29, 1997. This book contains the proceedings of that conference.