Asthma and chronic obstructive pulmonary disease (COPD) are major public health burdens. Asthma is characterized by airway inflammation, airway narrowing with reversibility, and hyperresponsiveness of airways. COPD has been associated with smoking and exposure to environmental fumes, which typically characterized by persistent airflow limitation and chronic inflammation of the airways. These differences are most apparent when young non-smoker with asthma and older smokers with COPD are compared. However, it would be difficult to differentiate asthma from COPD, especially in elderly who currently smoke or have a significant history of smoking. Furthermore, some patients exhibit characteristics of both diseases, this may represent a phenotype known as asthma-COPD overlap syndrome (ACOS). Therefore, the precise understanding of these diseases is important.