P-, E-, and L-selectin are structurally related membrane glycoproteins that initiate the attachment of flowing leukocytes to platelets, endothelial cells, or other leukocytes at sites of inflammation or tissue injury. This chapter focuses on P-selectin, the only selectin expressed on platelets. I will review aspects of its structure, the regulation of its expression, the mechanisms by which it mediates multicellular interactions under shear stress, and its role in physiological and pathological forms of inflammation and hemostasis.