Alpha 1 proteinase inhibitor, the archetypal member of the serpin superfamily is an acute phase protein and protects pulmonary tissues from gtatuitous elastase digestion and inhibits neutrophil elatase at sites of inflammation. In this book we have explained the structure function relationships of alpha 1 proteinase inhibitor. A detailed biochemical characterization and mechanism of multiple protease inhibition elucidated. Further, alpha 1 proteinase inhibitor interactions with negatively charged heparin have been localised in terms of specific residues involved and investigated in terms of both activation as well as conformational changes.