Medium Chain Triglycerides (MCT) are lipids which are commonly present in nature, for instance in milk and in some vegetable oils. The MCT molecule is composed of glycerol esterified with saturated linear-chain fatty acids containing 6 to 12 carbon atoms (MCFA: Medium Chain Fatty Acids: C(6:0), esanoic acid; C(8:0), octanoic acid; C(10:0), decanoic acid; C(12:0), dodecanoic acid). MCT for clinical use are currently obtained from vegetable fat through industrial processing. The large interest for MCT in clinical nutrition is due to unique physico-chemical, biochemical and metabolic features, which characterize MCT compared to conventional lipids (LCT: Long Chain Triglycerides). In particular, MCT represent a source of rapidly, fully available and ''high density'' energy, which makes their use very advantageous and suitable in a wide range of clinical situations, from the premature newborn to the adult patient. In addition to the MCT oil, known to physicians and nutritionists since the 1950s, and to the many commercially available MCT-containing compounds for oral-enteral use, in the last years it has become possible also to administer MCT-containing emulsions in parenteral nutrition. This has allowed to improve further knowledge of the effects of MCT, expanding the indications for use, often as a ''first-choice'' substrate in different types of critically ill patients.