In today’s world, the interest in traditional fermented foods and beverages is on the rise, especially after the recognition of health benefits of probiotics in recent years. Traditional fermented foods are consumed as healthy foods all over the world because of their biological activities, including anticarcinogenic, antibacterial, antioxidant, antiallergic, antiinflammatory, immunomodulatory, and antidiabetic activities. One such fermented food is kefir, which is a fermented beverage obtained from the milk of cow, sheep, and goat. Kefir is a complex mixture containing many useful bacteria and yeasts, proteins, sugars, fats, vitamins, minerals, and essential amino acids. In this chapter, the scientific data published about the history, production, health benefits, nutritional values, and microbiological and chemical properties of kefir will be discussed.