The usual question is: does green tea contain caffeine? The answer is yes, if it hasn't been decaffeinated. So does black tea, and oolong. A good post on teas in general and green tea in particular:
From alczap.thorne.com (Al Czap):
It is thought that the traditional use of tea (Camellia sinensis) began in China about 4700 years ago. Europeans were introduced to the beverage in the 1500's, and by the second half of the 17th century it was being widely consumed throughout Europe. Today, tea is the second most consumed beverage in the world (water is #1) with 2.5 million tons of tea leaves produced annually.
The Chinese produce over 300 varieties of tea, which can be separated into three basic categories; black tea, oolong tea, and green tea.
These three types of tea can actually be derived from the exact same plant. The difference between them is how the leaves are handled after harvesting. Black tea is allowed to ferment, and is then dried. Oolong tea is partially fermented. Green tea is dried without fermenting. Allowing the tea to ferment oxidizes naturally-occurring catechins, transforming them into theaflavins and thearubigin, chemicals responsible for the color and flavor of black tea. An increase in theaflavins increases the commercial value of black tea, but decreases the catechin content.
Green tea infusion contains intact catechin polyphenols, which give rise to its bitterness and astringency. Six catechin polyphenols have been isolated from green tea; (-)-epigallocatechin, (-)-epicatechin, (-)-epigallocatechin-3-O-gallate (EGCG), gallocatechin-3-O-gallate (GCG), methyl-epigallocatechin-3-O-gallate, and (-)-epicatechin-3-O-gallate (ECG). These substances were tested for their antioxidant activity, and the gallic acid esters EGCG and EGC were found to be the strongest antioxidants, with EGCG being over 200 times more active than Vitamin E in an in vitro model. In another test, EGCG was more active against fat rancidity (lipid peroxidation) than Vitamin C or Vitamin E, and also exhibited synergistic action with those vitamins.
Many nutritive and protective qualities have been associated with green tea, both in infusion and extract form. A Japanese epidemiological study of 9500 non-drinkers/non-smokers age 40 and above showed a decreased incidence of stroke (CVA) in those consuming green tea, with a direct correlation between increased consumption and decreased incidence, so that at 3-4 cups a day the overall incidence of CVA was 17 percent that of people drinking no tea. Other epidemiological studies show a decreased risk of esophageal, gastric, and colon neoplasms with tea consumption.
Numerous studies have shown that standardized green tea extracts or components of the extract exhibit antioxidant activity, stimulation of glutathione peroxidase and catalase, induction of phase II enzymes, and inhibition of cyclooxygenase, lipoxygenase, and angiotensin converting enzyme. Green tea extract also has anti-platelet-aggregation activity, and inhibits delta-amylase and sucrase, in addition to the known effects of catechin: collagen stabilization, histidine decarboxylase inhibition, and hepatic support.
Green tea extracts can contain a substantial amount of caffeine, and may be standardized to low levels of polyphenols. HPLC isolation and identification reveals that the main constituent of our extract is EGCG (epigallocatechin gallate), the most active compound in the extract.