Collagen constitutes 20 – 30 % of proteins in the organism of mammals and birds, and it is a major component of the intramuscular connective tissue. In the muscles, collagen is mainly stored in epimysium, perimysium, and endomysium. There are more than 20 genetic types of collagen in the skeletal muscles and, among them, collagen type I and type III are a significant portion. The morphology, composition, and quantity of the connective tissue in the muscles depend predominantly on their type, as well as on the species, breed, and age of the animal. Owing to differences in the methods of determining collagen, the content of this protein can differ in individual muscles. A high content of this incomplete protein in the connective tissue of the muscles has a significant impact on the tenderness of meat and decreases its quality. The cross-linking of collagen in the muscles that are highly active in live animals increases with age of animals and causes the meat to become hard. A lower content of collagen was found in the muscles with longer sarcomeres and in the meat from late maturing and castrated animals.