**1. Introduction**

Although different studies recognize the presence of asthma–chronic obstructive pulmonary disease (COPD) overlap syndrome (ACOS), the detection, diagnosis, and treatment of these patients in clinical practice are not always simple and are subject to different interpretations and controversies. These patients are of special interest, because they are usually excluded from clinical trials with new medications for asthma and also represent a clinically very important and quite prevalent population, apparently with particular characteristics: more respiratory symptoms, frequent exacerbations, and worse health-related quality of life [1–5]. They are also characterized by an increase in comorbidity and a greater consumption of health care resources compared to patients with only asthma or COPD alone. There are currently no universally accepted, validated criteria for the diagnosis of ACOS. Also, clinical trials are necessary to verify the response to treatments of this group of patients.
