Caffeine is a natural psychostimulant belonging to the methylxanthines and found in plants such as coffee, tea, cocoa and guarana. It is widely used both in nutrition and in dietary supplements, constituting one of the most studied and effective ergogenic substances worldwide. At the biochemical level, caffeine acts mainly as an antagonist of adenosine receptors in the central nervous system. Adenosine is a neurotransmitter that promotes relaxation and the feeling of fatigue; its inhibition by caffeine leads to increased alertness, reduced perception of fatigue and improved neuromuscular activation. At the same time, caffeine increases the release of catecholamines, such as adrenaline, enhancing the excitation of the nervous system. At the metabolic level, caffeine contributes to increased lipolysis, stimulating the release of free fatty acids from adipose tissue, which can be used as an energy substrate during exercise. In addition, it improves muscle contractility through increased availability of calcium ions within muscle fibers, which can lead to improved performance in both endurance and strength exercises. From an ergophysiological perspective, caffeine has been associated with improved endurance, increased power and strength, faster reaction times, and improved concentration. The reduction in perceived fatigue allows exercise to be performed at a higher intensity or for a longer duration. For this reason, it is a key ingredient in many pre-workout supplements. Overall, caffeine is a scientifically proven and highly effective ergogenic ingredient, with main actions being the stimulation of the central nervous system, the improvement of physical and mental performance and the support of energy metabolism. Its use is particularly widespread among athletes and individuals seeking increased alertness, performance and concentration, always with respect to dosage and individual tolerance.