Proliferative Kidney Disease (PKD) caused by the Malacosporean parasite Tetracapsuloides bryosalmonae, is the most economically damaging disease of British rainbow trout farming, costing the industry in excess of £2.5 million per annum. With no vaccine or prophylactic treatment available, and only management techniques currently adopted to minimise the stress and mortality associated with the disease, the research undertaken within this book addresses a potential alternative approach; selective breeding. The book will explain the investigation undertaken to identify that selective breeding for PKD resistance is possible, and how the level of additive genetic variation was calculated, allowing estimates of heritability to be documented for commercial strains of rainbow trout (Oncorhynchus mykiss).