Guanidinoacetic acid (GAA) is immediate substrate for biosynthesis of creatine (CREA). The phosphorylated form of CREA serves as a rapidly mobilisable reserve of high-energy phosphates in skeletal muscle to recycle adenosine triphosphate (ATP) and replenish cellular energy levels. Aside from improving energy utilisation, GAA possesses several features which further enhance its value as a feed additive for poultry. The feed supplement has good thermal stability, which allows it to be safely used in pelleted diets. GAA has high bioavailability, is cost-effective, and spares dietary arginine (ARG) and energy. Its digestion and absorption is similar to other amino acids, although there are some interactions. Supplementation with 0.6–1.2 g/kg can increase muscle CREA by 14–21%, respectively. Levels of utilisation (retained in the body vs. intake) of between 76.2% and 100% have been reported in arginine deficient diets, while this declines with higher arginine supplementation. Meta-analysis has shown that GAA supplementation improved FCR in a dose dependent and rather linear manner. Linear regression of the FCR data for GAA doses between 0.6 and 1.2 g/kg suggested 4.5 and 8.8 points improvement, respectively. The efficacious ARG sparing activity of GAA is an important feature, since birds are not able to synthesise ARG and are exclusively dependent on dietary sources of this amino acid. GAA has several other proven benefits, including improving fertility and semen quality, stimulating muscle growth through cell-signalling processes, and ameliorating bone growth and development. Optimal inclusion levels of 1.4 g/kg GAA have been reported for supporting fertility in birds.