Newcastle disease (ND) is causing greater economic losses to the poultry industry worldwide due to high mortality, decreased productivity in affected birds, increased costs involving in controlling disease and other associated factors.Newcastle disease virus (NDV) is a RNA virus belonging to the genus avula virus, family paramyxoviridae. This disease affects various species of birds including chicken, ducks, pigeons, turkeys, Japanese quail, emus, crows, sparrows, wild birds etc. Free living birds infected with NDV excrete larger quantities of the virus in their excretions and secretions and act as a source of infection to domestic poultry. This book emphasis the role of free living birds in the transmission of ND to domestic poultry.