The aim of this dissertation was to identify both patient related – and surgical factors influencing weight loss (WL) after bariatric surgery. By doing so optimization of postoperative results and standardization of the perioperative processes were emulated. Chapter 1 provides an introduction to the topic. Chapter 2 investigates patient-related factors contributing to postoperative WL in the preoperative phase. Results demonstrated that increased preoperative weight loss is associated with increased weight loss up to three years postoperatively in patients undergoing primary LRYGB. This chapter supports the hypothesis that patient motivation to adjust to a different lifestyle postoperatively is, albeit partially, quantified through preoperative WL. Chapter 3 demonstrates that structurally creating the gastric pouch with a gastric tube as calibration, leads to 5% more total weight loss after 2 year follow up compared to creating the pouch with the carpenters eye. Chapter 4 investigates whether patients demonstrating weight regain or insufficient weight loss after primary LRYGB would benefit from resizing the gastric pouch and which was the preferred technique. The conclusion was that in selected cases sleeve resection of the pouch and gastrojejunostomy without recreating the gastrojejunostomy may be the preferred technique over the technique that does recreate the gastrojejunostomy. Chapter 5 is a systematic review and meta-analysis aiming to research the possible influence of adherence to medical follow-up (FU) on postoperative WL both short and long term after LRYGB and LSG. This resulted in a significant association between adherence to FU and percentage excess weight loss (%EWL) up to three years after surgery. However, with the scarce available literature on the longer term FU (>3 years after surgery), no influence of FU on WL was discovered. Chapter 6 is a prospective cohort study and explores the association between adherence to FU and WL in patients scheduled for 3 to 5 year individual follow-up appointments. No such association could be demonstrated. Age, persistent comorbidities and vitamin deficiencies, a yearly salary