Asthma and chronic obstructive pulmonary disease (COPD) as separate diseases are considered for a long time. Now this concept needs to be re-evaluated because in many clinical situations both diseases may coexist, or one condition may evolve into the other. Especially in older patients with obstructive disease an overlapping diagnosis of asthma and COPD may be observed. Epidemiological data show that overlap syndrome is recognized by the coexistence of incompletely reversible airway obstruction and features of airway remodeling. The analysis of clinical symptoms indicates that the risk factors are the same in asthma and COPD, and a role of inflammatory process in the small airways may be a key point to understand the pathology of overlap syndrome. Studying these events may bring new insights into the treatment of obstructive diseases of the respiratory tract.