Chronic obstructive pulmonary disease (COPD) and asthma are two different diseases that affect the airways. Around one-third of COPD and asthma patients have features of both asthma and COPD and thus are diagnostically labeled as asthma-COPD overlap (ACO). ACO patients experience worse symptoms and more serious respiratory attacks than those with COPD or asthma alone, but we do not know why. To address these questions, our study will investigate the underlying inflammatory mechanisms in the airways of patients with ACO. We will collect tissue samples and cells from the airways of volunteers with ACO using a technique called bronchoscopy and perform genomics on these samples. These data will enable us to identify the key airway features of ACO. We will also use this cohort to determine which features of ACO lead to a good therapeutic response from inhaled corticosteroids, a class of medications used in COPD and asthma. We will use high-resolution imaging techniques to investigate how inflammation relates to persistent changes in the structure of the airways and the lungs. Our research will reveal the disease mechanisms of ACO so that we can better diagnose and prescribe the most effective therapies for ACO in clinical practice.