Growth hormone (GH) and prolactin (PRL) are anterior pituitary hormones that have multiple roles in growth and metabolism. Both hormones are important in mammary development and breast cancer. The epidermal growth factor (EGF) family of peptides and the receptors that they activate (the ErbB family) are also major players in mammary biology and pathophysiology. Recent studies in signal transduction have highlighted the interplay between signaling pathways referred to as crosstalk. In this review, cell biological and signaling studies related to crosstalk between GH and PRL and the ErbB family are discussed. In particular, the role of GH- and PRL-induced phosphorylation of ErbB receptors in regulating EGF responsiveness is highlighted with attention to potential pathophysiological relevance. © Springer Science+Business Media, LLC 2008.