Fibroblast growth factor 21 (FGF21), a metabolic hormone predominantly produced by the liver, is also expressed in adipocytes and the pancreas. It regulates glucose and lipid metabolism through pleiotropic actions in these tissues and the brain. In mice, fasting leads to increased PPAR-α mediated expression of FGF21 in the liver where it stimulates gluconeogenesis, fatty acid oxidation, and ketogenesis, as an adaptive response to fasting and starvation. In the fed state, FGF21 acts as an autocrine factor in adipocytes, regulating the activity of PPAR-γ through a feed-forward loop mechanism. Administration of recombinant FGF21 has been shown to confer multiple metabolic benefits on insulin sensitivity, blood glucose, lipid profile and body weight in obese mice and diabetic monkeys, without mitogenic or other side effects. Such findings highlight the potential role of FGF21 as a therapeutic agent for obesity-related medical conditions. However, in human studies, high circulating FGF21 levels are found in obesity and its related cardiometabolic disorders including the metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease and coronary artery disease. These findings may indicate the presence of FGF21 resistance or compensatory responses to the underlying metabolic stress, and imply the need for supraphysiological doses of FGF21 to achieve therapeutic efficacy. On the other hand, serum FGF21 has been implicated as a potential biomarker for the early detection of these cardiometabolic disorders. This review summarizes recent developments in the understanding of FGF21, from physiological and clinical perspectives.