Weight gain is one of the more common side effects of psychotropic drugs (neuroleptics, antidepressants, lithium, benzodiazepines). This effect depends on the type of drug, doses and length of treatment. It appears gradually and seems linked to the patient's clinical history. Among neuroleptics, phenothiazines and benzamides have the most important effect on weight. Weight gain with lithium is well documented and it does not regress with time. There is actually a controversy about antidepressants direct effects on weight. Current information concerning benzodiazepines is insufficient. Weight gain is not an unavoidable side effect and etiopathogenic mechanisms are not well known. Many hypothesis are discussed (biochemical, pharmacological, psychological and environmental). Weight gain is a complex and multifactorial phenomena.