Allergy is an untoward physiologic response in host which is acquired by immunologic mechanisms. Allergic diseases are one of the most common chronic diseases in children and teenagers and have increased since the latter half of the 20th century around the world. The rapid increase of allergic diseases were also reported during the 1980s in Korea. Environmental changes play an important part in the development of allergic diseases, although genetic factor is fundamentally involved in the pathogenesis of allergy. Many studies have explained that allergy is the process in which it divides into production of chemical mediators by attachment of allergen to mast cell and hyper-responsiveness of the target organ by interaction with chemical mediators. However, although many diagnostic methods and treatment sources have been developed from these concepts, the mechanism of the allergy is not yet fully understood. Several researchers and societies have attempted to make guidelines for the diagnosis and treatment of allergic diseases. However, it is difficult to set guidelines, because children frequently suffer from variable non-allergic diseases which show symptoms similar to allergy. Therefore, it is helpful to approach allergic diseases in children by their age group. We suggest approaches for the diagnosis and management of bronchial asthma, allergic rhinitis and atopic dermatitis which are common in children according to their age group.