Increasing rates of type 2 diabetes (T2DM) follow the obesity ‘epidemic’, with 86% of patients with T2DM being overweight and over half being obese. Literature has highlighted that being overweight or obese increases the risk of diabetes. Weight loss for obese patients is associated with clinical improvements, although this evidence is mostly from short-term studies. As part of a Health Technology Assessment systematic review the long-term (≥2 years) effects of weight loss on change in diabetes-related outcome measures for those with diabetes, or risk of developing diabetes for those without diabetes, was investigated in obese individuals. Eleven studies published between 1966 and 2001 fulfilled the inclusion criteria (Caucasian, BMI >28 kg/m2, adults, no eating disorders, weight loss and changes in diabetes-outcome measures). Results of these studies indicated that intentional weight loss reduces the risk of developing diabetes in the long term and those participants with T2DM often have reduced clinical symptoms and mortality risk. These results have been verified and enhanced by literature published since this review. A similar systematic review was conducted as part of a six-phase project, the PRevent Obesity GRowing Economic Synthesis Study. This review excluded BMI >34 kg/m2 and was restricted to lifestyle interventions (or intentional weight loss). Limited information relating to diabetes was gained, with only a non-significant increasing trend for mortality from diabetes for severe weight cycling practices being suggested. Other results indicated a relationship between weight loss and fasting plasma glucose, but because of the heterogeneity of participation groups and lack of definition in relation to diabetes this relationship was not formalised. In summary, weight loss is beneficial for long-term diabetes outcomes for overweight, obese and morbidly-obese participants. There is little research evidence for those individuals who are overweight or just obese, indicating areas of future research in terms of prevention of both obesity and diabetes.