The prevalence of obesity and diabetes has been dramatically increasing during the last decade suggesting a greater patient need for more efficacious and safer drugs. Large molecule therapy has played an important role in diabetes since the discovery of insulin. This legacy was continued upon the introduction of Humulin (first recombinant insulin), Humalog (first engineered insulin) and Byetta (first incretin mimetic). Several other protein therapeutics, such as leptin, adiponectin, bone morphogenic protein-9 and others, are currently in or considered for therapeutic development. Among them, FGF21 is one of the most promising candidates given its outstanding pharmacologic benefits for nearly each and every abnormality of a metabolic disease and lack of apparent side effects in a variety of animal models. Thus, FGF21 represents a novel and appealing therapeutic reagent for Type 2 diabetes mellitus, obesity, dyslipidemia, cardiovascular and fatty liver diseases. The in vitro biology, genetic animal models and in vivo pharmacology of FGF21 will be discussed in this chapter.