Overexpression and activation of ErbB2 receptor tyrosine kinase in breast cancer is strongly linked to an aggressive disease with high potential for invasion and metastasis. In addition to inducing very aggressive, metastatic cancer, ErbB2 activation mediates processes such as increased cancer cell proliferation and survival and is needed for normal physiological activities, such as heart function and development of the nervous system. How does ErbB2 activation make cancer cells invasive and when? Comprehensive understanding of the cellular mechanisms leading to ErbB2-induced malignant processes is necessary for answering these questions. Here we present current knowledge about the invasion-promoting function of ErbB2 and the mechanisms involved in it. Obtaining detailed information about the "bad" behavior of ErbB2 can facilitate development of novel treatments against ErbB2-positive cancers.