Caffeine is a naturally-occurring compound in tea (L-theanine is another). The caffeine content in a cup of tea is much less than that in a cup of coffee, but the exact amount of caffeine in tea is dependent upon a variety of factors. These factors include tea growing conditions, soil, plucking standards, steeping time and steeping temperature. Caffeine levels are also different for different types of tea. Herbals and tisanes, with a few exceptions (such as Yerba Maté), generally contain no caffeine.
Concentration of Caffeine in Coffee vs. Tea
Is the caffeine content in tea a health concern?
According to overwhelming scientific evidence, there are no adverse health effects linked to caffeine when it is consumed in moderation (≤400 mg/about 8 cups of blact tea per day). In fact, caffeine can improve physical performance by enhancing muscle contractions, improving mood, increasing concentration and mental awareness, boosting fat burning, and reducing fatigue.
At the same time, drinking tea is also linked to feelings of relaxation and decreased stress. This is largely because of the L-theanine component of tea.
*chart and caffeine information taken from Tea, Nutrition and Health: Myths and Truths for the Layman (June 2012 World Tea Expo)