Technical Abstract: Pneumonic pasteurellosis is the major economic problem of the cattle industry in North America. This disease is characterized by an acute, severe, fibrinonecrotic pleuropneumonia. Pasteurella haemolytica A1 is commonly isolated from these pneumonic lesions. It has been demonstrated that stress or viral infection compromises defense mechanisms of the upper respiratory tract and lung, predisposing to an initial multiplication of bacteria in the nasopharynx and, subsequently, lungs are deluged with large numbers of bacteria. Once multiplication in the alveoli has begun, virulence factors exert their influence to induce an excessive host inflammatory response that results in severe tissue damage. Despite a large number of studies conducted to explore the complex interaction between P. haemolytica and the host response, there still remains a lack of detailed understanding. This review discusses evidence of the role of the main virulence factors of P. haemolytica on the pathogenesis of pulmonary damage.