The clinical course--and hence the prognosis--of patients suffering from malignant tumors are essentially determined by the capability of tumor cells to metastasize. During the past decade knowledge about genetic aberrations, as well as molecular and cell biological mechanisms which are involved in the regulation of tumor metastasis, has dramatically increased and consequently led to the development of new theoretical and experimental strategies in cancer treatment. The objective of this review is not only to give an overview about the principal cell biological and molecular mechanisms of tumor metastasis, but also to discuss potential therapeutical options resulting from this knowledge.