Skeletal muscles as an active hormonal compartment in the response of physical activity secrete substances named myokines capable of modulating metabolic processes. Myokines take part in communication between muscles and other tissues. Irisin (Ir) - a newly discovered adipomyokine - is cleaved and secreted to the circulation from a fibronectin type III domain containing protein 5 (FNDC5). The mechanism of Ir action has not been described precisely, and receptors for the molecule are not defined yet, but it has been proposed to promote browning of white adipose tissue into beige fat cells. To date we have distinguished two types of adipose tissue in mammals - white, which not only functions as a store of energy but also can play a pro-inflammatory role (secreting adipokines), and brown adipose tissue. Brown adipose tissue has a high mitochondrial content and can dissipate chemical energy in the form of heat (nonshivering termogenesis). It plays a natural antiobesity role and protects against obesity-related diseases. The development of beige adipose tissue, which in its structure and function is similar to brown adipose tissue, and the possibility to modify its amount through some external factors, are nowadays among the most important targets of research on fat cell biology.