Respiratory disease is one of the most serious disease complexes affecting beef cattle production. For example, it is claimed to cost the UK industry about 70 million pounds per year. It is usually associated with young cattle and can occur in a variety of situations. It is a good example of multifactorial disease in that its aetiology involves both infection by a variety of microorganisms and a number of environmental factors. Several distinct syndromes occur and a number of microorganisms are thought to be important including the bacteria Pasteurella haemolytica type A1, P. multocida, Haemophilus somnus, Corynebacterium pyogenes, Mycoplasma bovis and M. dispar. Of the viruses, bovine herpes virus 1 (BHV1) and respiratory syncytial virus (RSV) are known to be important, the former also causing the specific syndrome, infectious bovine rhinotracheitis (IBR) in addition to its involvement in the pneumonia complex. Other viruses of possible importance include para-influenza 3 (Pi3), adenoviruses, bovine viral diarrhoea (BVD) virus, coronavirus and rhinovirus.