Bariatric surgery such as Roux-en-Y gastric bypass (RYGB) is increasingly performed in obese patients with type 2 diabetes (T2DM) due to its beneficial metabolic effect. RYGB is distinct from non-bariatric surgery as it improves glycaemic control immediately post-surgery. Most bariatric centres also use a low calorie diet preoperatively which also impacts on glycaemic control before surgery. Glycaemic management of these patients therefore needs to be reviewed perioperatively to avoid hypoglycaemia. To date, no study has assessed how best to manage this group of patients preoperatively and postoperatively. GLUCOSURG-pre and GLUCOSURG-post studies were designed to assess the effect of intensive glucose management before and after RYGB on glycaemic outcome. Moreover, patients with difficult controlled diabetes are more at risk of complications of diabetes. Given the rapid improvement in glucose control following RYGB, its effect on microvascular complications needs to be assessed. In this thesis, I measured the changes in diabetic nephropathy, retinopathy and neuropathy; and compared the changes in nephropathy and retinopathy to a control group. My study showed that 3 months of intensive management of glycaemia before surgery, or the first 2 weeks after surgery had not resulted in better glycaemic control at 1 year. RYGB has substantial effects on glucose control, and additional intensive glucose-lowering interventions do not confer clinical benefits compared to conservative approaches. In the case-control study, RYGB patients showed substantial reductions in albuminuria, while the rates of retinopathy progression were similar to those observed in a medically treated group. There was no change in peripheral nerve function 1 year after RYGB surgery. My study was limited by small sample size and short duration of follow up. Nonetheless, the result would reaffirm the importance of annual surveillance for diabetic retinopathy and neuropathy after RYGB.