The taxonomy of the family Pasteurellaceae Pohl 1981 appears to be as complex as that of Enterobacteriaceae. 16S rRNA sequencing indicates that the family should be divided into more than 20 genera. According to phylogenetic investigations, the genus Pasteurella sensu stricto includes three subclusters, two of which represent taxa mainly associated with avian hosts. True species of the genera Actinobacillus and Haemophilus have not been reported from birds. Several new taxa, which have been shown to belong to the family Pasteurellaceae Pohl 1981 have been reported from birds. Some of these seem to represent genus-like structures. Due to a high degree of host-specificity observed for many taxa belonging to the family, the existence of many more species can be foreseen as more avian species are examined. The pathogenesis of Pasteurella infections in birds is poorly understood. However, it has long been recognized that the severity of the disease and its incidence may vary considerably depending on several factors associated with the host, the environment or the bacterial strain. Several virulence factors of P. multocida, may be of importance for infection of birds and are discussed. There is some evidence that the capsule of P. multocida is of importance in the protection against phagocytosis by immunocompetent cells and that it mediates resistance to complement. Endotoxin is another factor which has persistently been associated with pathogenicity of P. multocida. There are some indications that the P. multocida exotoxin (PMT), which is involved in the pathogenesis of atrophic rhinitis in swine, may play a role in some arian infections. High molecular weight outer membrane proteins of P. multocida have been speculated to be the main iron acquisition system of arian strains of P. multocida and thereby representing virulence factors. An outer membrane protein with anti-phagocytic activity has also been demonstrated. The potential role of different enzymes in the pathogenesis P. multocida infections has been investigated, but not conclusively demonstrated. Apart from the knowledge concerning the PMT encoding gene toxA, very little is known about the genetic basis of diseases caused by pasteurellas associated with arian species.