The serine protease thrombin is well recognized as being pivotal to the maintenance of hemostasis under both normal and pathological conditions. Its cellular actions are mediated through a unique family of protease-activated receptors (PARs). These receptors represent a novel family of G protein-coupled receptors that undergo proteolytic cleavage of their amino terminus and subsequent autoactivation by a tethered peptide ligand. This paper reviews the consequences of PAR activation in thrombosis, vascular injury, inflammation, tissue injury, and within the tumor microenvironment.