The metabolic syndrome (MS) has a high prevalence in different countries (∼ 10% in France). It associates in the same subject central obesity, decrease in HDL cholesterol, increase in plasma triglycérides, glucose intolerance and hypertension. Insulin-resistance is a common feature even though it has been excluded from the more recent definitions because of the difficulty to assess it in clinical practice. Physiopathology of MS is not univocal. Stress, insulin resistance, central obesity are concerned. Marine omega 3 fatty acids (EPA and DHA) have demonstrated in many studies in rodents and some studies in humans their potentiality to prevent and treat MS, by their insulin-sensitizing effect, their ability to modulate mental stress response, to decrease plasma triglycerides, to reduce lipotoxicity, to slow down atherogenesis, to decrease oxidative stress and to ameliorate endothelial dysfunction. Additional studies are required to definitively confirm their usefulness and define their optimal daily amount for prevention and treatment beside physical activity, maintenance or reduction of body weight and pharmacological therapy of specific components of MS.