Caffeine is naturally occurring in the tea plant, known as Camellia sinensis, and will be found across all tea categories: white, green, oolong, black & fermented. Caffeine and its associated compounds are used as the tea plant's powerful and natural defense against insects.
Before brewing, dry tea leaves actually contain about the same amount of caffeine as coffee beans by weight. However, a brewed cup of tea has less caffeine than coffee - usually about 1/3 to 1/2 the amount found in drip coffee. There is typically somewhere between 25-50 mg of caffeine in one 8 ounce cup of tea.