Green tea is made from the leaves of the Camellia sinensis L plant, which is rich in polyphenol catechins and caffeine. There is increasing interest in the potential role of green tea extract (GTE) in fat metabolism and its influence on health and exercise performance. A number of studies have observed positive effects of GTE on fat metabolism at rest and during exercise, following both shorter and longer term intake. However, overall, the literature is inconclusive. The fact that not all studies observed effects may be related to differences in study designs, GTE bioavailability, and variation of the measurement (fat oxidation). In addition, the precise mechanisms of GTE in the human body that increase fat oxidation are unclear. The often-cited in vitro catechol-O-methyltransferase mechanism is used to explain the changes in substrate metabolism with little in vivo evidence to support it. Also, changes in expression of fat metabolism genes with longer term GTE intake have been implicated at rest and with exercise training, including the upregulation of fat metabolism enzyme gene expression in the skeletal muscle and downregulation of adipogenic genes in the liver. The exact molecular signaling that activates changes to fat metabolism gene expression is unclear but may be driven by PPAR-γ coactivator 1-α and PPARs. However, to date, evidence from human studies to support these adaptations is lacking. Clearly, more studies have to be performed to elucidate the effects of GTE on fat metabolism as well as improve our understanding of the underlying mechanisms.