It is well appreciated that thrombin as well as other proteases can act as signaling molecules that specifically regulate cells by cleaving and activating members of a novel class of protease-activated receptors (PARs). The utility of gene knockout strategies to define and better comprehend the physiological role of specific proteins is perhaps best exemplified in the field of thrombin receptors. The development of PAR knockout mice has provided the unique opportunity to identify and characterize new members of this novel family of GPCRs, evaluate the interaction of PARs jointly expressed in common cells and tissues, and better understand the role of PARs in thrombosis, restenosis, vascular remodeling, angiogenesis, and inflammation. Presently, 4 members of the PAR family have been cloned and identified. In this review, we examine experimental evidence gleaned from PAR-/- mouse models as well as how the use of PAR-/- mice has provided insights toward understanding the physiological role of thrombin in cells of the vascular system and vascular pathology.