L-Theanine is an amino acid found in the leaves of tea (Camellia sinensis).
It's more to green tea than black tea, but even more in white tea.
Theanine is a derivative of L-glutamine. Only the L-enantiomer is in the natural state. It penetrates the blood-brain barrier and can have a calming and invigorating effect.
It is also an antioxidant and stimulates the immune system.