The purpose of this cross-sectional, observational study is to evaluate the site and mechanism(s) for expiratory airflow limitation in chronic, treated, current or former smokers (\>15 pack years) with COPD, Emphysema, and Asthma-COPD Overlap with mild to severe expiratory airflow limitation. Treatment may include short and long acting inhaled beta2agonists, short and long acting inhaled muscarinic receptor antagonists, inhaled and or oral corticosteroid, oral antibiotic, supplemental oxygen, and PDE type 4 inhibitor. In some cases, the patient may have had a history of asthma preceding the development of COPD (Asthma COPD Overlap).