STAT transcription factors were discovered 10 years ago as mediators of interferon-induced gene expression. They now form an important group, comprising seven members, that are activated by virtually every cytokine and growth factor. Their critical role in development and normal cell signaling has been largely determined through the analysis of transgenic mice lacking individual STAT genes. In addition, cell culture work has further delineated their importance in cellular transformation, apoptosis, differentiation and growth control. This review discusses the specific phenotypes of STAT-deficient animals with a focus on STAT5 and STAT3, as these two STAT molecules are required for normal breast development and involution, respectively, and may play an important role in breast carcinogenesis.