Obstructive pulmonary diseases are a group of respiratory conditions characterized by airflow limitation. They are prevalent in the population and lead to significant morbidity and mortality. The two most commonly encountered conditions are asthma and Chronic Obstructive Pulmonary Disease (COPD). Although, for many years, the two conditions were regarded as separate, it was realized that some patients may have an overlap between these illnesses. This observation led to the identification of asthma-COPD overlap (ACO) syndrome. ACO is an umbrella term that is used “to collectively describe patients who have persistent airflow limitation together with clinical features that are consistent with both asthma and COPD”. Its importance in the field of respiratory medicine is increasing every year as there are more data that underline the importance of its timely diagnosis and management. The current review is a timely update of the advances in our understanding of ACO.