Green tea (GT) is made from the leaves of the Camellia sinensis L plant which is rich in polyphenol catechins and caffeine. For some time now there has been an increasing interest in the capacity of GT to oxidise fats and promote weight loss which in turn may improve body composition, health and exercise performance. As a result, GT has become a popular dietary supplement on the sports nutrition market, especially as a fat burner and weight loss tool. Long term GT intake, in some but not all cases, has been shown to promote weight loss. It is believed that the anti-obesity effects of GTE intake may be attributed to elevated fat oxidation and total energy expenditure. In support of this, it has been shown that in a number of cases, GT intake (short term and long term) at rest and during exercise may increase energy expenditure and fat oxidation. Although several studies have observed positive effects the literature is inconclusive. The precise dose, duration of intake, and population that elicit maximal effects are currently unknown. In addition the bioavailability of GT catechins in vivo and the subsequent bioactivity is less well understood. In humans, the bioavailability of GT following intake determines the bioactivity. GT catechins (parent compounds) are extensively metabolised (conjugated compounds) in the gut and liver, poorly absorbed and are found in low concentrations in vivo. The emergence of ľomic technology is a new technique that can identify the interaction between the metabolic effects of GT and the association to the specific GT catechins following ingestion in vivo. This firstly will advance our understanding of GT bioavailability and bioactivity, but also provide support and comparison to the putative in vitro mechanisms. At present the precise mechanisms of GT and site of action are unclear. However the mechanisms behind the effects of long term GT may be different than those that explain the short term effects of GT. Short term intake of GT is thought to inhibit catechol-O-methyltransferase (COMT) in vitro, while caffeine is thought to inhibit phosphodiesterase (PDE) in vitro. Both of these mechanisms may act synergistically to increase lipolysis. Alternatively long term GT intake may involve changes in expression of specific fat metabolism genes, which have been supported only by animal data. Both mechanisms lack the use of GT catechin compounds and concentrations similar to what is observed in vivo as well as supporting human studies. Therefore the mechanisms of GT remain speculative. While GT may offer promising benefits to body composition, health and exercise performance, the lack of consistent evidence means that practical application of GT intake is currently not possible. Despite this, GT supplements are frequently used.