This chapter provides an overview of the classification, description, hosts, key developments, diagnostic techniques, and diagnostic reagents for the transmissible turkey enteritis virus. It is a highly infectious agent causing depression, diarrhea, and anorexia in all ages of turkeys, but young stocks are more severely affected. The main host for the virus are turkeys. The virus had not occurred in Western Europe until very recently. Clinical entity of the virus was first described in 1951. Diagnostic techniques for the virus include fluorescent antibody, virus neutralization, and electron microscopy. A cell-mediated immunity in vitro technique has recently been found for the detection of the virus.