Quantification of carnitine is required to diagnose carnitine deficiency. The radioenzyme method which was used in most early studies is briefly explained, but this method has not been used recently. Now, tandem-mass spectroscopy is used for carnitine deficiency in inherited metabolic diseases because this method can quantify and identify acylcarnitine species. High-performance liquid chromatography is used only in special laboratory. The enzyme cycling method can measure free carnitine and total carnitine, and acylcarnitine is calculated. This method is very useful for daily clinical practice because it can be used in the automatic biochemistry analyzer. However, unfortunately the use of this method is limited only in Japan. The outlines of pharmacokinetics of carnitine are very important for appropriate administration of carnitine. The distribution, bioavailability, and excretion are explained in this chapter.