The formation of placenta can be viewed as a controlled process of inflammation or cancer development in which cell proliferation, invasion, and immune surveillance or evasion are promoted in a time- and space-specific manner. This suggests that the functions of prostasin are mainly active in pathophysiological conditions, more so than those in basic or normal physiological conditions. The uniqueness of the prostasin serine protease is its membrane anchorage via a glycosylphosphatidylinositol moiety. This anchorage exposes the entire prostasin molecule to the outside while still being immobilized on the cell membrane and gives prostasin a better chance to be used as a biomarker or drug target. As presented in placenta development, the equilibrium between growth and differentiation dictates healthy and disease states. This book provides readers with an integrated knowledge on serine protease prostasin, starting with its discovery, protein isolation, cDNA, and gene cloning, to its functions in physiology and pathophysiology, and to its application in the development of therapeutics. This is the first-of-its-kind book that covers the history, present state of knowledge, and future perspectives of the prototypic glycosylphosphatidylinositol-anchored epithelial extracellular serine protease prostasin, which is a key player in the regulation and maintenance of the extracellular fluid homeostasis. The chapters encapsulate all that has been learned about prostasin in the technical sophistication, but more importantly, also highlight a critical analysis on the problems with the current state of research and outline the challenges for the future.