In chickens and in animals in general, growth is influenced by genotype as well as by environmental factors including nutrition. Hormonal changes form, in part, the causal link between these interacting factors and the physiological processes leading to growth. It is well established that growth-related hormones (GH, SS) play different roles in different tissues. Several studies have investigated the nutritional and/or genetic effects on growth in chickens, leading to the general conclusion that the improvement of growth in modern broiler chickens is largely influenced by the genotype. In addition, it has been shown that the nutrition-determined regulation of tissues is also genotype-specific.