Egg yolk immunoglobulin (IgY) is a major immunoglobulin from birds, amphibians and reptiles. IgY does not activate mammalian complement system, and does not bind to mammalian rheumatoid factors, protein A or G. In addition, it has a high yield, and is easy to extract and purify. With these advantages, IgY has been widely used in veterinary science, functional food and bio-products, and proved potential application for disease control and treatment. This paper reviews biological characteristics of IgY, its extraction, purification, and application in immunodiagnosis and treatment.