Most of us associate caffeine with coffee, but tea can actually be a very rich source of caffeine as well. Caffeine is found in many different plants where it functions as a pesticide and wards of plant eating insects. Coca beans, leaves from the Yerba mate shrub, and berries from the Guaraná tree are three examples of other comparatively rich sources of caffeine.
The amount of caffeine in a cup of tea will vary greatly depending on how the tea has been prepared. Generally speaking, black tea has the highest caffeine content, followed by oolong tea. Both of these the types are allowed to oxidize for a long time.
Green tea is much less oxidized than black tea and oolong tea, and will therefore have lower caffeine content. When creating white tea, the aim is to halt the oxidation process as rapidly as possible, and a high-quality white tea will therefore have a caffeine content below even green tea. There are however exceptions to these general rules, and green Gyokuro tea from Japan will for instance contain more caffeine than most black teas.
To ingest 200 mg of caffeine by drinking standard coffee, you typically need to drink 16 fluid ounces (4.73 deciliters). If you instead go for a strong espresso made from robusta beans, 5 fluid ounces (1.47 deciliters) will be enough. This can be compared to black tea, where you must drink at least 40 fluid ounces (1.18 liters) to ingest 200 mg of caffeine. If you choose green tea, you will need to throw back 80 fluid ounces (2.36 liters) to get the same effect. These figures are however only rules of thumb; the caffeine content in coffee as well as in tea will depend on a wide range of factors and can vary significantly. It should also be noted that tea often contains more caffeine than coffee, if you compare the dry forms instead of prepared brews.