Ghrelin and the ghrelin receptor (GHSR1a) play a central role in growth hormone secretion, food intake, energy metabolism, and other important functions. This book summarizes the present situation of the GHSR1a gene polymorphisms and their genetic effects on growth and fatty acid component traits in domestic animals, humans and rodents. It highlights a unique molecular evolution of the GHSR1a gene among animal species, and its significant genetic and epistatic effects on carcass and fatty acid component traits in a sex-dependent fashion. The volume also shows the overdominance effect of the GHSR1a-DelR242 locus on growth and its molecular mechanisms and the central role of the bovine GHSR1a_5'UTR-(TG) repeat locus in growth and fatty acid component traits in cattle.