Carnitine functions as a carrier of acyl-group of long-chain fatty acids into mitochondria and facilitates β-oxidation. In this process, three enzymes and a transporter constitute carnitine cycle (also called carnitine shuttle). The interesting history of research of these molecules, including carnitine palmitoyltransferase (CPT1 and 2), carnitine acylcarnitine translocase (CACT), and carnitine transporter (OCTN2), reveals stories of inquisitive minds solving the mystery of how acylcarnitine passes through mitochondrial outer and inner membranes. Carnitine cycle is the core of carnitine functions, so understanding its history might facilitate the understanding of carnitine function described in this book.