Carnitine in the human body is derived from the intake of preformed dietary carnitine and biosynthesized carnitine, stemming from the metabolism of lysine and methionine. Carnitine is synthesized in liver and kidney, stored in skeletal muscle, and excreted mainly in urine. Carnitine has two main functions, i.e., transporting long-chain fatty acids into the mitochondrial matrix for beta-oxidation to provide cellular energy and modulating the rise in intramitochondrial acyl-CoA/CoA ratio, which relieves the inhibition of many intramitochondrial enzymes involving glucose and amino acid catabolism. Thus, the main consequence of carnitine deficiency is impaired energy metabolism. Human carnitine deficiency can be either hereditary or acquired. Hereditary carnitine deficiency can be grouped into three clinical entities: myopathic carnitine deficiency, systemic carnitine deficiency, and organic acidurias. Acquired carnitine deficiency is due to inadequate intake, increased requirement, and increased loss of carnitine. The definite diagnosis of carnitine deficiency is based on the determination of free- and acylcarnitine levels in serum, urine, and/or tissues. The estimated safe and adequate daily carnitine intake for adults is 150-500 mumol/day whereas pharmacological doses of carnitine are required for the treatment of hereditary carnitine deficiency.