Since the third edition was published in 1992 there have been many more developments in the poultry industry, which have made a new edition necessary. These include new systems which are environmentally and welfare 'friendly'; much more emphasis on hygiene and vaccines in disease control and the role of molecular biology and cloning; further advances in management with special reference to feeding, drinking, lighting and egg collection; advances in genetics; developments in housing; nutritional advances; and increasingly restrictive legislation concerning the safety to the public of poultry products (particularly zoonotic diseases and drug residues) and welfare.