The discovery that platelets express CD40 and the CD40 ligand has transformed these cells, once seen as exclusively involved in coagulation and thrombosis, into active players of immunity and inflammatory injury. Many of the broad and potent biological activities mediated through the CD40/CD40 pathway by immune and nonimmune cells are also exerted by activated platelets. This occurs either through the constitutive expression of CD40 on the platelet surface or the activation-induced expression of the CD40 ligand, which is membrane bound and released from the surface in a soluble form. The most prominent activities mediated by the platelet CD40/CD40 ligand pathway include inflammatory, immunoregulatory, and hemostatic functions, all of which contribute to the newly expanded view of platelets as key biological mediators involved in disease processes such as atherosclerosis, inflammatory bowel disease, and diabetes. Therefore, considering platelet CD40 and CD40 ligand as novel biological targets is justified and supported by animal studies. The clinical profit to be gained from blocking this molecular pair will be determined by results in humans with conditions in which the platelet CD40/CD40 ligand pathway is crucially involved in disease pathogenesis.