Caffeine occurs naturally in the leaves, seeds or fruit of over 100 plant species, including the Camellia Sinensis from which tea is derived. Other common sources include coffee, cocoa beans & cola nuts. It’s also added to many energy drinks & some pharmaceutical products.
Despite popular myth, tea does have less caffeine per serving than coffee & can be drunk in greater quantities without adverse effects.
A typical cup of black tea (190mls) contains 40-50mg of caffeine: about 1/2 the amount in the same size cup of instant coffee (75mg of caffeine) & around one third the caffeine in freshly brewed or percolated coffee (100+ mg of caffeine). Less fermented teas generally contain significantly less caffeine than black teas.