To understand carnitine deficiency and insufficiency, knowledge of homeostasis of carnitine is essential. Overview of carnitine distribution in the body and homeostasis is provided. Then, factors which influence carnitine homeostasis, such as tissue distribution, dietary intake, biosynthesis, excretion, and reabsorption, are explained in detail. The collapse of one of these factors causes carnitine deficiency and insufficiency in various pathophysiological conditions and medical treatments such as dialysis and drugs.