The main threat and the reason for most cancer deaths are not the primary neoplasias, but secondary tumors, the metastases. Drastic phenotypic and biochemical changes occur during the metamorphosis of a normal tissue cell into an invasive cancer cell. These alterations concern various areas such as growth factor signaling, cell-cell adhesion, gene expression, motility or cell shape. Cancer cells of epithelial origin can even shed their typical qualities and characteristics and adopt a mesenchymal-like phenotype. This is often referred to as an epithelial-mesenchymal transition. Various oncogenes, tumor suppressor genes and metastasis suppressor genes are known to affect the invasiveness and the metastatic potential of tumor cells. Cells of the innate and adaptive immunity, adjacent stroma cells as well as chemokines and their receptors also play a vital role in the spread of cancer cells. Furthermore, the micro-environment, vascularization and the supply with special cytokines affect the above-mentioned changes. Finally, some researchers claim that tumors consist of two types of cells - transit amplifying cells and cancer stem cells. Only the latter are thought to be able to proliferate indefinitely and thus they might be the cells that successfully spread and initially build most of the clinically relevant metastases. This review article describes some of the molecular principles which underlie those changes as well as covers some aspects of current research.