The debate about whether asthma and chronic obstructive pulmonary disease (COPD) are distinct clinical syndromes is not new; there is heightened interest in understanding the group of individuals with obstructive lung disease who seem to have elements of both conditions because recent studies have demonstrated increased risk for respiratory events and exacerbations. We describe the clinical characteristics of this subtype of disease and suggest 4 working definitions of individuals who would fall into the asthma–COPD overlap category. Understanding the mechanisms underlying these subtypes will hopefully lead into a better understanding of therapeutic strategies that can target specific pathobiologic pathways.