This article surveys elastinolytic proteinases in man, excluding enzymes of the pancreas and digestive tract. Special emphasis is placed on the elastase of polymorphonuclear neutrophils (PMN). The properties of this latter enzyme, its target molecules in plasma and connective tissues, and its endogenous regulators are briefly discussed. Persistent activity of the enzyme, even in the presence of its regulatory inhibitors, is explained. The chapter closes with a brief discussion of several pulmonary diseases in which elastase-mediated tissue injury is thought to play a role.