From the manufacturer's website:
L-theanine is a unique amino acid that is found almost exclusively in the tea plant (Camellia sinensis) and which has been used safely in Japan for decades. L-theanine has been shown to positively modify brain waves and the key neurotransmitters involved in mood, focus, and memory.* L-theanine enhances the production of the neurotransmitters dopamine and serotonin, and also appears to play a role in the formation of GABA – a neurotransmitter that acts like a "brake" during times of stress.* L-theanine also enhances brain alpha-wave activity, a marker of relaxation.*