Carnitine is an essential molecule for mitochondrial beta-oxidation of long-chain fatty acids and other cellular functions. Several rare, inherited disorders of carnitine metabolism occur in humans, and secondary carnitine deficiency is an important feature in a variety of clinical settings. Many of these conditions can be detected via quantitative analysis of free and esterified carnitine in plasma or urine, which thus offers an effective means for assessing the transport and initial processing of fatty acids. Here, we describe some of the methods most commonly employed for quantification of plasma carnitine and consider some of the advantages and disadvantages of these approaches. © 2019 by John Wiley & Sons, Inc.