This chapter describes the properties of extracellular proteases produced by Pseudomonas aeruginosa and examines their possible roles in various P. aeruginosa infections. Elastin is an insoluble fibrous constituent of connective tissue resistant to digestion by common proteolytic enzymes and also to heat, acids, alkalis, and various denaturing agents. The elastase production of P. aeruginosa can easily be determined using nutrient agars containing elastin powder, where a zone of clearing can be observed surrounding colonies producing the enzyme. The alkaline proteinase is active against various denatured protein substrates, but is inactive against both elastin and native collagen. Pseudomonas proteases can elicit hemorrhages in internal organs or tissues of animals according to the route of P. aeruginosa inoculation. Keratitis produced by the infection with P. aeruginosa is often difficult to treat, progresses rapidly, and may lead to extensive corneal scarring and corneal perforation.