Bariatric surgery is the most effective treatment of obesity and its associated diseases like type 2 diabetes mellitus. Given the obesity epidemic and the efficacy of surgical treatment, the number of surgical weight loss procedures has grown in recent years. Nevertheless, there is little consensus regarding the extent of preoperative investigations required prior to patients undergoing surgery. This article aims to discuss the available evidence on which preoperative tests are useful for the detection and treatment of conditions such as venous thromboembolism, obstructive sleep apnea syndrome and Helicobacter pylori-positive gastritis prior to an operation. The present literature suggests that only a few preoperative investigations are essential, but that preoperative multidisciplinary care is beneficial.