Most clinicians can easily distinguish asthma and COPD. Asthma usually has an early onset with intermittent symptoms, a good response to inhaled therapy, and is often associated with other allergic diseases, whereas COPD is of late onset, slowly progressive symptoms, poor response to inhaled therapy, and is usually associated with long-term smoking. However, patients can sometimes have features of both diseases, and this condition has been termed asthma-COPD overlap syndrome (ACOS). Some overlap may be predicted because asthma and COPD are both common, and there is no evidence that one disease protects against the other. To call this overlap a syndrome is misleading, however; it includes different phenotypes, such as patients with COPD and eosinophilic inflammation, patients with asthma and severe disease or who smoke in whom there is predominantly neutrophilic inflammation, and patients with asthma who have largely irreversible airway obstruction due to structural changes. Thus, it may be better to refer to asthma-COPD overlap (ACO), rather than ACOS. Indeed, the patients who receive a primary diagnosis of asthma who have some features of COPD are better considered as phenotypes of asthma. The “Dutch hypothesis,” first proposed in the 1960s, suggested that there was a common genetic background to airway obstruction with a spectrum of clinical disease from asthma to COPD. Although the validity of the Dutch hypothesis has been fiercely debated, recent genetic studies indicate that there is very little, if any, common genetic background of asthma and COPD. ACO is now recognized in several national and international guidelines.