The selectins: E-selectin, P-selectin, and L-selectin are adhesion molecules that are crucial for binding of circulating leukocytes to vascular endothelium during the inflammatory response to injury or infection. Accumulated evidence indicates that selectins regulate adhesion of circulating cancer cells to the walls of blood vessels. Selectin ligands are transmembrane glycoproteins expressed on leukocytes and cancer cells that promote bond formations with selectins to mediate inflammatory processes. Selectins and selectin ligands also participate in signal transduction to regulate diverse cellular functions. Sialyl Lewis X (sLe^x) and sialyl Lewis A (sLe^a) tetrasaccharides are carbohydrate motifs displayed on protein or lipid scaffolds that are critical components of functional selectin ligands. Selectin binding to sLe^x and sLe^a present on colon, gastric, bladder, pancreatic, breast, and prostate carcinomas enhances distant organ metastasis. High expression of sialyl Lewis ligands on these cancers is significantly correlated with a poor post-operative prognosis. This review will focus on the roles of E-selectin and P-selectin in cancer progression. Understanding the role of selectins in cancer supports the development of novel selectin-based therapies to control metastasis.