L-Theanine in isolation was not only found to have several effects on CNS, blood Flow, and heart rate but also to have an antagonistic effect on caffeine when administered in combination by counteracting arousing effects and lowering the blood pressure. L-Theanine influenced different mood and cognitive dimensions, especially improved reaction time, long- and short-term memory and suppression of distraction. However, due to the setup of those studies, it cannot be clearly said that effects such as reduced stress, increased relaxation, and decreased anxiety are only the result of L-Theanine alone, but probably of caffeine or the combination of caffeine and L-Theanine. Insufficient evidence was available on the psychopharmacological effects of EGCG. It was mainly found to counteract increased oxidative stress, down regulate psychological a stress response, block progressive increases of systolic blood pressures, and modulate GABAergic systems. It is not possible to draw a conclusion regarding effective doses and effects on specific mood and cognitive dimensions. Caffeine was found to modulate the activity of many neurotransmitter systems, particularly to antagonize adenosine receptors resulting in increased activity within the ascending cholinergic activating system, while affecting the CNS and Cardiovascular activity. Moreover, caffeine increases alpha-frequency and blood pressure and decreases the resting state cerebral blood flow. Caffeine primarily influenced attention, psychomotor, and memory variables, but had little effect on mood. The Majority of mood effects were found for subjective global mood ratings. Based on the reviewed literature it can be stated that green tea phytochemicals have an important effect on mood and cognition. This can be the basis for designing novel products containing effective and safe quantities of these compounds. While this systematic review found a reasonable amount of evidence that both EGCG and LTheanine exhibit neuroprotective activity and that L-Theanine (and to a lesser degree of EGCG) influence mood, the extracted data suggests that the various improvements I cognitive faculties linked to green tea consumption are not the consequence of a single component. The improvements in cognitive tasks in the reviewed studies are strongly linked to the presence of both caffeine and L-Theanine. Further research is needed to establish the minimal dosage of green tea or green tea components to reliably elicit either acute or chronic effects.