The development and progression of tumors result from the concerted activity not only of tumor cells with neighboring cells e.g., fibroblasts and inflammatory cells. Host-tumor interactions are considered critical in tumor invasion and metastasis. In vitro studies as well as established in vivo models have analysed the reciprocal effects of tumor-host interactions for the tumor invasion process. These studies have shown that modifications in the extracellular matrix composition surrounding the tumors as well as alterations in the expression of tumor cell receptors or in the expression of growth factors/cytokines and proteases, are critical regulators of a developing tumor. We shortly review the most important and well characterized mechanisms involved in the progression of tumor cells through tissues, especially those participating in cellular communication, cell adhesion, and proteolysis.