Casein is an animal protein that occurs naturally in milk and constitutes approximately 80% of its total protein content. It is a protein of high biological value, with a complete amino acid profile, including all the essential amino acids required for the synthesis and maintenance of muscle mass. In the field of sports nutrition, it is widely used as a protein supplement, mainly due to its particular physicochemical and metabolic behavior. At a biochemical level, casein differs significantly from other milk proteins, such as whey, as during digestion it forms a semi-solid gel in the acidic environment of the stomach. This property slows down gastric emptying and leads to a slow and gradual release of amino acids into the blood, which can last up to 6–8 hours. As a result, casein contributes to maintaining stable levels of amino acids in the plasma for an extended period of time. The main physiological role of casein is related to its anti-catabolic effect. The prolonged supply of amino acids reduces muscle proteolysis and helps maintain a positive nitrogen balance, especially during periods of fasting, such as overnight sleep or phases of caloric deficit. Although it activates the anabolic mTOR pathway to a lesser extent compared to whey, its overall effect on the preservation of muscle mass is particularly important. Casein also contains high levels of glutamine, proline and casein phosphopeptides (CPPs), which help improve the absorption of minerals such as calcium and phosphorus, while supporting bone health. In addition, its slow digestion enhances the feeling of satiety, which makes it particularly useful in fat loss programs. It is mainly found on the market as micellar casein, which preserves the natural micellar structure of the protein and is considered the highest quality form, as it ensures maximum slow absorption. Overall, casein is a scientifically proven protein supplement, with main advantages in anti-catabolic action, prolonged supply of amino acids and support of muscle mass and recovery, especially when consumed before bedtime or during periods of prolonged food abstinence.