Avian viral diseases such as avian influenza, Newcastle disease, Gumboro, Marek's among others cause huge economic losses in the poultry industry both in the developed and developing countries. The current treatment and vaccination methods are not effective especially in a developing country setting. This book looks at the candidate gene approach in order to understand functional polymorphisms and their role towards disease resistance in line with the genetic background of commercial, indigenous and wild chicken populations. The polymorphisms of two genes are studied in depth; the BLB2 gene in the MHC region and the Mx gene. It is hoped that the insights garnered from this book will add value towards developing a genetic control strategy of viral diseases.