Eczema is a general term for a group of inflammatory skin diseases whose etiology is still unclear and may be caused by a variety of internal or external factors. The medical diagnosis of eczema is based primarily on clinical manifestations. Eczema is only a morphologically descriptive name and is not an etiologic diagnosis. Eczema is formed by a combination of intrinsic genetic abnormalities and environmental factors. There is a definite relationship between eczema and heredity, but this relationship is quite complex and may be polygenic; it is not the case that if a parent has the disease, the child must have it. Various factors in the external environment such as chemicals, food, drugs, light, skin trauma, infection, microorganisms, etc. can cause lesions in eczema patients. Internal diseases such as liver and kidney diseases, nutritional deficiencies, psychological factors, etc. can also trigger or aggravate eczema. The cause of eczema may be different in each case, making it one of the most difficult diseases to identify clinically. Allergic reactions, also known as allergic reactions, are an important cause of eczema. There are a variety of substances in the environment that can trigger an abnormal immune response in a sensitive person, known as an allergic reaction. The substance that causes an allergic reaction (allergic response) is called an allergen (allergen). In addition to allergic reactions, there are many substances in the environment that can trigger dermatitis through non-immune mechanisms, and these substances are called irritants. Acute irritation by strong irritants in the environment, such as strong acids, bases, and corrosives, and chronic and persistent irritation by weak irritants such as water and detergents can produce eczema lesions in anyone’s skin, independent of genetics. It is important to recognize that eczema is only a morphologic diagnosis, not an etiologic one. It is prompted by the need to actively search for the cause of eczema, and only when the cause is found can a cure be possible. For example, in a case initially diagnosed as eczema, treatment for many years was temporarily effective and could not be cured. Later, after careful search for the etiology, it was found to be allergic contact dermatitis caused by exposure to an external substance, which was then cured.