L-Theanine is a type of amino acid that is the main component of green tea leaves (Camellia sinensis). It’s known for its relaxing effects on both humans and animals. Here we look at how it works - and particularly its role in glutamate and gamma-aminobutyric acid (GABA). These are the major neurotransmitters in the mammalian brain and inhibitory GABA and excitatory glutamate work together to control many processes, including the brain’s overall level of excitation.