Due to its high effectiveness in treating obesity, and because of growing number of overweight patients, bariatric surgery has been increasingly popular lately. The World Health Organization sounds a warning about obesity as the most dangerous of chronic diseases which does not tend to recede spontaneously and has alarming consequences not only in terms of physical health but also for psychological and social functioning. Therefore, obesity shall definitely be treated. Nowadays, bariatric surgery is considered to be the most effective method of treating morbid obesity. A meticulous diagnosis and assessment of psychological condition of morbid obesity patients is essential for the effective treatment. It allows better understanding of mechanisms contributing to obesity in the operated patients, and selection of appropriate interventions aiming at increasing effectiveness of bariatric surgery.