About 70 years ago, Peyton Rous described the progression of cancer towards metastasis formation as "the process whereby tumors go from bad to worse". The interactions of tumor cells with endothelium are pivotal steps in this process. This review focuses on the role played by the selectins and their ligands in these interactions and especially in tumor cell extravasation. The working hypothesis of researchers studying tumor cell extravasation is that the tumor cells follow the extravasation strategy of leukocytes in their migration towards inflammatory sites. A significant portion of this review is, therefore, dedicated to the molecular mechanisms underlying leukocyte extravasation and to a comparison between the extravasation strategy employed by leukocytes and tumor cells. The review also summarizes some of the available data on signals generated by selectin-selectin ligand interactions.