Asthma and chronic obstructive pulmonary disease (COPD) are highly prevalent chronic diseases in the general population. Both are characterized by heterogeneous chronic airway inflammation and airway obstruction. In both conditions, chronic inflammation affects the whole respiratory tract, from central to peripheral airways, with different inflammatory cells recruited, different mediators produced, and thus differing responses to therapy. Airway obstruction is typically intermittent and reversible in asthma but is progressive and largely irreversible in COPD. However, there is a considerable pathologic and functional overlap between these 2 heterogeneous disorders, particularly among the elderly, who may have components of both diseases (asthma-COPD overlap syndrome). The definitions for asthma and COPD recommended by current guidelines are useful but limited because they do not illustrate the full spectrum of obstructive airway diseases that is encountered in clinical practice. Defining asthma and COPD as separate entities neglects a considerable proportion of patients with overlapping features and is largely based on expert opinion rather than on the best current evidence. The presence of different phenotypes or components of obstructive airway diseases, therefore, needs to be addressed to individualize and optimize treatment to achieve the best effect with the fewest side effects for the patient. Although specific interventions vary by disease, the treatment goals of obstructive airway diseases are similar and driven primarily by the need to control symptoms, optimize health status, and prevent exacerbations.