Functional amino acids are the special amino acids which have special functions besides protein synthesis. They are necessary not only to animal growth and maintenance requirement, but also to the synthesis of bioactive molecules. Remarkably, arginine and its precursor NCG can enhance conception rates, embryogenesis to improve fertility, fetal and postnatal growth, and neonatal health. Guanidino acetic acid improves the breeding performance of layers and the production performance of broilers significantly.