The metabolic syndrome (MetS) is a cluster of metabolic abnormalities including abdominal obesity, glucose intolerance, hypertension and dyslipidaemia and is associated with an increased risk of vascular events. Since the initial description of the MetS, several expert groups produced different definitions. This variability led to confusion and absence of comparability between studies. Although there is agreement that the MetS is a major public health challenge worldwide and consistent evidence stresses the need for intervention, the definition of the syndrome remains a matter of debate. This review considers the different definitions of the MetS. These include those proposed by the World Health Organisation, the European Group for the Study of Insulin Resistance, the National Cholesterol Education Program Adult Treatment Panel III, the American College of Endocrinology and American Association of Clinical Endocrinologists and the latest International Diabetes Federation definition which includes ethnic-specific waist circumference cut-off points. These definitions share several features but also include important differences; all have limitations. Selected (after a Medline search) studies comparing the different definitions are also considered. There is a need for a standardised definition of the MetS. Furthermore, a definition tailored for children and adolescents is essential. Prospective long-term studies are needed to validate the prognostic power of these definitions. As new information becomes available the definition of the MetS might be further modified.