Asthma and COPD, once regarded as distinct disease entities, often overlaps especially in the elderly and smokers. GINA/GOLD joint document 2014 has proposed a clinical entity of ACOS (asthma-COPD overlap syndrome) characterized by irreversible airflow limitation with clinical features of both asthma and COPD, although evidences for definite diagnosis and treatment are limited. ACOS includes several different phenotypes such as severe asthma with airway remodeling, incomplete airflow reversibility as a consequence of childhood asthma and smoking, eosinophilic phenotype of COPD, and so on. Considering the therapeutic modalities currently available for asthma and COPD, it is important to identify the patients who respond well to inhaled corticosteroids.