Competitions in combat sports are divided into certain weight classes in order to compete fairly among athletes of similar body weight. However, athletes lose weight compulsorily or tactically (to encounter weaker opponents, to be between two weights, to be faster and more agile, etc.). In many scientific studies, it is seen that the athletes lose their weight close to the competition scale, and generally they lose a lot of weight on the last day. In addition, it is stated that the rate of weight loss varies between 2-10% of the body weight of the athletes. Many methods are used to set weight before the competition scale. These; Reducing energy intake, reducing fluid consumption, not consuming anything especially the day before the competition scale, increasing the duration and intensity of exercise, loss of fluid with intense sweating, use of sauna, use of diuretic drugs. As the use of these harmful methods and the rate of weight loss increase, their negative effects on physical, physiological, psychological and performance increase at the same rate. In this study, studies in the literature on the effects of weight loss on strength, endurance, anaerobic performance, reaction and balance performance will be examined and the causes of possible effects will be determined.