As a high-quality, nutritious food, milk is widely loved by people. β-casein is an important protein component in milk and is one of the indicators for evaluating the quality of milk. It plays an important role in protecting human health. As there are many varieties of β-casein in milk, it is of great significance to define the role of each β-casein. The most common variants of β-casein in milk are A1 and A2, while the other types are rare. Therefore, the current research regarding cow milk β-casein mainly focuses on these two variants. In this paper, the varieties and allele frequency of milk β-casein and its relationship with human health are elucidated. Recent progress in studying β-casein in milk is reviewed in order to provide readers with insights into milk β-casein.