Proteases refer to a group of enzymes whose catalytic function is to hydrolyze(cleavage) peptide bonds of proteins. Proteases occur naturally in all organisms and are involved in a multitude of physiological reactions from simple digestion of food proteins to highly regulated cascades. In this book, the authors discuss the maturation and stabilization mechanisms of two hyperthermostable subtilisin-like serine proteases from thermococcus kodakarensis; serine protease-related innate immunity; fungal and bacterial serine proteases; and the physiological function and pathology of neurosin/kallikrein 6 in the central nervous system.