Mitochondrial beta-oxidation of fatty acids is an important source of energy for the cells, especially during fasting. Since 1973 several inherited defects in beta-oxidation have been described. Defects in mitochondrial beta-oxidation are one of the largest groups of inborn errors of metabolism.This review article is based on the experience of the authors and on literature studies. The authors' experience is from laboratory diagnostics and clinical experience in the departments of medical biochemistry and peadiatrics at our hospital.Beta-oxidation defects are potentially fatal disorders. Symptoms are usually seen during fasting, e.g. during childhood infections. Organs which preferably oxidize fatty acids or ketone bodies are especially vulnerable. Often, but not always, the patients have hypoketotic hypoglycaemia. In addition one can see affection of the liver, heart, muscular and nervous systems. The diseases can manifest both in childhood and adulthood and are often less severe in adulthood. The main principles of symptomatic treatment are avoidance of fasting and regular intake of a low-fat, high-carbohydrate diet. The diagnosis can be difficult to establish, especially in asymptomatic phases.