The heterocyclic amines (HCAs) are a family of mutagenic/carcinogenic compounds found in cooked meats. Several HCAs are mammary gland carcinogens in rats. Of these compounds, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the major one present in the human diet. This report reviews the studies on rat mammary gland carcinogenesis by HCAs; discusses what is currently known regarding mechanisms of mammary gland carcinogenesis of PhIP, especially the significance of metabolic processing; and further highlights the evidence for the possible role of PhIP in human breast cancer. Environ. Mol. Mutagen. 39:165–170, 2002. Published 2002 Wiley-Liss, Inc.