Irisin is a proteolytic hormone released into circulation by skeletal muscle tissue during acute exercise, and is a derivative of the cleaved plasma membrane protein Fibronectin type III domain containing protein 5 (FNDC5). Found in muscle tissue, FNDC5 is synthesized at increased levels during exercise as a result of the overexpression of the exercise-responsive transcriptional co-activator PGC-1α (peroxisome proliferator-activated receptor-γ co-activator-1α). Like its parent polypeptide, Irisin can induce the browning of subcutaneous adipocytes, or the conversion of white adipose tissue (WAT or white fat) into brown (or beige) adipose tissue (BAT or brown fat). Given that brown fat can undergo thermogenesis, or the physiologic process of heat production, Irisin contributes to the metabolic process by increasing thermogenic function and glucose homeostasis. Irisin, thus, represents a link between exercise and “burning” of fats and/or sugars, and consequently may have relevance in the development of new treatments for diabetes and obesity.