Protein is perhaps the most reactive of the major food components. During food processing, the essential amino acids, lysine, tryptophan, methionine and cyst(e)ine, may react with other food components causing a loss in amino acid bioavailability and sometimes a reduction in the digestibility of the whole protein molecule. This review first discusses some recent developments concerning protein-polyphenol reactions, racemization and lysinoalanine formation, and then describes the reactions of proteins and reducing sugars (the Maillard reaction) in greater detail. We report on the chemistry of the Maillard reaction, the nutritional and physiological properties of the newly-formed products and their metabolic transit in the rat. In practice, the Maillard reaction is by far the most important reaction of food proteins. It is especially important in milk products since these are the only naturally-occurring protein foods with a high content of reducing sugar. Lysine is the most sensitive amino acid to damage during processing and storage and its losses may be of nutritional significance to certain population groups, such as babies, who are often dependent on a single manufactured product as their sole source of nourishment.