Loss of appetite is a common problem in many diseases. In various chronic diseases,loss of appetite is reinforced by its impact on the patient's quality of life and prognosis. It is necessary to understand the mechanisms that stimulate appetite, the pathophysiology caused by the chronic disease and the way it leads to a decrease in appetite, the effect of various treatments on the subject and the psychological aspect which is sometimes also a leading cause. In addition, in order to deal with the problem first, we must be familiar with the tools at our disposal to identify a decrease in appetite. Covered here are the physiological mechanisms that regulate appetite, the importance of food in the healthy and ill person, the reasons leading to its decline, the existing assessment tools for assessing the patient's appetite, and the nutritional implications of prognosis and quality of life. Nutritional support, pharmacological treatments, complementary treatments and the advantages and disadvantages of each method.