Polycystic ovary syndrome (PCOS) is a highly prevalent endocrine disorder that is primarily characterised by a hyperandrogenic state and ovulatory disturbances and which is associated with multiple metabolic abnormalities and a high prevalence of excess weight or obesity, particularly of the abdominal phenotype. This chapter reviews the role of obesity in the pathophysiology of PCOS and summarizes all major aspects of lifestyle intervention. The prevalence of obesity in PCOS appears to be much greater than that expected in the general population, ranging from 30% to 70% depending on ethnicity, geographical areas, environmental factors, genetic susceptibility and other still unknown factors. Infertility is associated with earlier onset obesity, and menstrual disorders are frequent with the onset of excess weight during puberty. Guidelines for the treatment of obesity can be applied, probably without any substantial specific changes, to the treatment of overweight and obese women with PCOS.