With the exception of a few consistent pathogens--Pasteurella multocida strains of bovine hemorrhagic septicemia and fowl cholera, Actinobacillus (Haemophilus) pleuropneumoniae, Haemophilus aegyptius and Haemophilus paragallinarum--members of the family Pasteurellaceae are commensal parasites on mucous membranes of vertebrate animals. Many have pathogenic potential, which becomes manifest under conditions of immunodeficiency and stress. Pathogenesis (except in porcine atrophic rhinitis) depends on mobilization of inflammatory responses probably in large part by endotoxin with contributions from protein toxins, which interfere with leukocyte activity and, by their cytotoxicity, cause exacerbation of the inflammatory reaction. Disease patterns include pneumonic/septicemic, upper respiratory and local/traumatic. Acquired resistance is chiefly antibody-dependent, and, with current and emerging biotechnical resources, stands a good chance of being artificially achievable for many important diseases attributed to Pasteurellaceae.