Approximately 90% of all cancer deaths arise from the metastatic spread of primary tumours. Of all the processes involved in carcinogenesis, local invasion and the formation of metastases are clinically the most relevant, but they are the least well understood at the molecular level. Revealing their mechanisms is one of the main challenges for exploratory and applied cancer research. Recent experimental progress has identified a number of molecular pathways and cellular mechanisms that underlie the multistage process of metastasis formation: these include tumour invasion, tumour-cell dissemination through the bloodstream or the lymphatic system, colonization of distant organs and, finally, fatal outgrowth of metastases.