Coughs are usually divided into 3 categories according to their duration: acute cough, subacute cough and chronic cough. Acute cough lasts less than 3 weeks, subacute cough 3 to 8 weeks, and chronic cough greater than or equal to 8 weeks. 1. Acute cough: The common cold is the most common cause of acute cough. Other causes include acute bronchitis, acute sinusitis, allergic rhinitis, acute attacks of chronic bronchitis, and bronchial asthma. 2. Subacute cough: The most common causes are post-cold cough, bacterial sinusitis, asthma, etc. 3. Chronic cough: Chronic cough has more causes and can usually be divided into two categories: (1) those with definite lesions on initial X-ray chest radiographs, such as pneumonia, tuberculosis, lung cancer, etc.; (2) those with no obvious abnormalities on X-ray chest radiographs and cough as the main or only symptom, which is usually referred to as unexplained chronic cough. The common causes of chronic cough are: cough variant asthma, postnasal drip syndrome, eosinophilic bronchitis and gastro-oesophageal reflux cough, which account for 95 to 95% of chronic cough in respiratory medicine outpatient clinics. Other etiologies are less common but are widely involved, such as chronic bronchitis, bronchiectasis, endobronchial tuberculosis, allergic cough, and psychogenic cough.