Poultry is an excellent source of animal protein to fulfil the needs of human demands in terms of egg and meat. The poultry industry often experiences the losses due to various diseases among which Infectious Bursal Disease (IBD) and Ranikhet disease (RD) have become more significant as intensive way of poultry production is followed. The control of these diseases can be achieved by suitable immunization methods and strict biosecurity measures. Outbreak of IBD and RD diseases occur as a result of faulty vaccination schedules, using low titred vaccine and improper managemental practices. This book, a part of M.V.Sc. thesis submitted to Tamil Nadu Veterinary and Animal Sciences University, Chennai, India and information provided in this book will be helpful for the poultry entrepreneurs to strengthening of flock immunity and biosecurity measures to overcome the problem of disease outbreaks.