We first published the “Important Poultry Diseases ” in 1972 and, ever since, it has remained the most sought after of all our publications. However, in the intervening years, new causes of disease such as chicken anemia virus and pneumovirus were identified. Therefore, Intervet Research teams developed new vaccines to combat the changed pattern of diseases. Many of these are inactivated vaccines, in either single or combined form, resulting in new vaccination programs. We have, therefore, produced this fourth edition of the handbook to cover these developments. It is intended as a pocket reference book for people working with poultry. It is not highly academic as it is written from a practical angle to assist with everyday problems in the field.