In this chapter, the chemical structure, salts, and physical properties of carnitine are described. Carnitine is called vitamin because it was discovered as vitamin BT for some species of insects, but not vitamin in humans. Carnitine transports long-chain fatty acid into inside of mitochondria, then the long-chain acyl group in long-chain fatty acid bound with CoA, forming acyl-CoA such as palmitoyl-CoA. This acyl-CoA is oxidized in β-oxidation and produces large amounts of ATP. This process is very important in tissues which require large amounts of ATP such as heart and skeletal muscle. Basic understanding of what is acyl-groups, what is CoA, and basic knowledge of energy metabolism are explained. After that, biochemical and physiological functions of carnitine and how carnitine is important in energy metabolism are described. These knowledge will provide good understanding of why carnitine deficiency and insufficiency cause various symptoms such as hypoglycemia, hyperammonemia, cardiac dysfunction, etc., which are described throughout this book.