Infectious diseases, particularly those caused by viral pathogens, pose one of the most serious threats to the growth and sustainability of the global poultry industry. The industry relies heavily on vaccination-based control strategies, but there are important challenges for this approach, such as the immunosuppressive effects of some viral pathogens and the increasing diversity of pathogens. This chapter reviews viral diseases affecting poultry, including the four most significant viral diseases in terms of their widespread distribution, effect on avian health and role in producing economic losses. For each disease, symptoms, diagnosis and control measures are discussed. The major threats to vaccination-based strategies are reviewed, and some potential responses are considered.