Teas containing more leaf buds tend to contain more caffeine, whereas teas made of mature leaves and stems tend to contain less caffeine. The processing also affects caffeine content; some roasted teas, such as hojicha, are very low in caffeine.
The teas with the most caffeine are thus “tippy” teas, those with the most leaf buds. These include Silver Needle or Bai Hao Yinzhen (a white tea), Tippy Assam, Yunnan Gold, and other tippy black teas, and a number of tippy green teas. Tippy Assam is often used in Irish Breakfast and other strong breakfast blends, so these blends can tend to be high in caffeine.
It is a widespread myth that black tea contains the most caffeine of any tea. Black teas made of larger, more mature leaves, such as Lapsang Souchong, can be very low in caffeine.
Caffeine content also depends on brewing. Longer steeping times and using more leaf both result in a higher caffeine content. Ways of brewing tea that result in a stronger tasting cup usually result in greater caffeine extraction.