Although major progress has been achieved in treating breast cancer patients, metastatic breast cancer still remains a deadly disease. A full understanding of the process of systemic cancer cell dissemination is therefore critical to develop next generation therapies. A plethora of experimental data points toward a central role of an epithelial to mesenchymal transition (EMT) in the multistep cascade of metastasis formation. However, in patients the data are based on correlative studies which often, but not always, tie the expression of EMT markers to cancer invasion, metastasis and poor clinical outcome. Moreover, the notion that cancer cells are able to switch between different modes of migration asks for a thorough review of the actual relevance of EMT in cancer metastasis.