A brief review of processes occurring in fat cells is given and the regulatory effects of certain hormones and nutrients are presented. Fat cells are found in the white adipose tissue and store substantial amounts of triglycerides. The process through which triglycerides are formed in these cells is called lipogenesis and is stimulated by insulin. The opposite process, lipolysis, i.e. the degradation of triglycerides with the liberation of glycerol and free fatty acids into the circulation, is stimulated by glucagon and adrenaline. Many factors related to nutrition such as, for example, fasting, nutritional status, the carbohydrate level of the diet, amount and composition of fats in it, can modify these processes. The diet and its composition also have an important effect on brown adipose tissue.