This chapter describes several assays useful for studying the various facets of tissue factor pathway inhibitor (TFPI) function, discusses procedures for the purification of TFPI from mammalian cell culture, and summarizes the properties of the TFPI molecule. A unique feature of TFPI is its factor Xa-dependent inhibition of factor VIIa-tissue factor activity, and the most reliable assays for identifying TFPI are assays that measure this inhibition. The generation of polyclonal and monoclonal antibodies to TFPI has allowed for the identification of TFPI using immunoassays and Western blot analysis. TFPI functions as a multivalent protease inhibitor; it inhibits activated factor X (Xa) directly and inhibits factor VIIa-tissue factor activity in a factor Xa-dependent fashion by forming an inhibitory complex consisting of factor Xa, TFPI, factor VIIa, and tissue factor. These activities suggest that TFPI serves a regulatory role in hemostasis. The chapter also describes the purification and characterization of the tissue factor pathway inhibitor.