Johne's disease is a chronic, granulomatous enteritis of ruminants and some wild-type species that is caused by infection with Mycobacterium avium spp. paratuberculosis (MAP). This inflammatory bowel disease (IBD) is characterised by a lengthy yet variable asymptomatic subclinical phase in which infected animals can transmit MAP to their progeny and herd mates; this is followed by a clinical phase characterised by chronic diarrhoea, wasting, and eventually death. The impact of Johne's disease is particularly evident in the dairy and beef industries, where significant losses due to reduced productivity and fertility, mortality, and premature culling have been reported. This book addresses the genetics of this condition and its possible treatments.