Eosinophils play an important role in the development of allergic diseases. This paper aims to describe the relationships between eosinophils and allergic diseases in terms of eosinophil maturation，migration， activation，degranulation and delayed apoptosis，elaborating their role in three different allergic diseases. By study⁃ ing the biological characteristics of eosinophils and their roles in allergic diseases，we can find more directions for the treatment of allergic diseases in order to guide the clinical diagnosis.