First, the definition of carnitine deficiency and carnitine insufficiency is explained in detail. Both are considered as carnitine deficiency, but there are some differences, and they sometimes mix up in clinical practice. Carnitine supplementation is the only way to treat carnitine deficiency and insufficiency. Then, classification of carnitine deficiency and insufficiency by etiology is described. Carnitine deficiency and insufficiency manifest as various symptoms such as hypoglycemia, hyperammonemia, fatigue, muscle weakness and cramps, cardiac hypertrophy and dysfunction, arrhythmia, etc. The detailed mechanisms why carnitine deficiency and insufficiency cause such various symptoms are explained. There are several attempts to establish animal model of carnitine deficiency, so they are introduced briefly.