Understanding the molecular mechanisms of invasion and metastasis is crucial for the development of novel therapeutic strategies to treat metastases. Considerable studies revealed that the establishment of metastasis is the final outcome of a series of processes such as tumor growth, angiogenesis, tumor cell detachment and invasion of extracellular matrix. Each step of this multistep process is essential for tumor cell survival and establishment of secondary lesions and is regulated by interactions of tumor cells with host microenvironment. This review is focused on the molecular mechanisms involved in the regulation of this processes. The central role of adhesion molecules and ECM degrading proteinases in disruption of cell-cell and cell-ECM associations as well as degradation of extracellular matrix and basement membranes is emphasized.