Bacteria play a critical role in the severe pneumonia and fatalities associated with the bovine respiratory disease complex. Although numerous bacteria have the potential to cause pneumonia, only a small number of these are responsible for the majority of cases, of disease. Virulence and immunogenic characteristics of these organisms are important determinants of the host response to infection. These bacterial characteristics are reviewed and applied to a discussion of the epidemiology, pathogenesis, and prevention of bacterial pneumonia is also discussed.