Not all children with wheezing symptoms in children ≤5 years of age are asthmatic. In this age group, the diagnosis of asthma is based primarily on clinical judgment and must be evaluated periodically as the child grows. The diagnosis of early childhood asthma is made primarily with the help of clinical judgment (symptoms and physical examination). Because of the important clinical implications of making the diagnosis of “asthma” in children with wheezing symptoms, it should be differentiated from other causes that may cause persistent or recurrent wheezing symptoms. Episodes of wheezing and coughing also frequently occur in non-asthmatic children (especially under 3 years of age). Conditions that may cause wheezing in children ≤5 years of age include: (1) Transient early wheezing: seen only in children up to 3 years of age. It is associated with prematurity and parental smoking. (2) Persistent early wheezing (before 3 years of age): Children with recurrent episodes of wheezing following an acute viral respiratory infection, with no evidence of atopy, no family history of allergic reactions, and unlikely to develop late-onset wheezing/asthma within the next decade. Most children have symptoms that persist until 12 years of age. wheezing before 2 years of age is primarily associated with respiratory syncytial virus infection, while older preschoolers are associated with other viral infections. (3) Late-onset wheezing/asthma: These children have asthma throughout their school years and into adulthood. Most of them have atopic signs (atopy), often with eczema, and airways with the pathology of asthma. The following symptoms were highly suggestive of an asthma diagnosis: frequent episodes of wheezing symptoms, more than once a month; activity-induced cough or wheezing; nocturnal cough in the absence of viral infections; wheezing without seasonal variation and persistent symptoms after age 3 years. The possible development of asthma in older children could be predicted based on a simple clinical indicator of the presence of wheezing by age 3 years, the presence of one major risk factor (parent with asthma or eczema), or the presence of two of the three minor risk factors (elevated blood eosinophils, wheezing in the absence of a cold, and allergic rhinitis). However, inhaled glucocorticoids have failed to prevent the development of asthma in these children. Other diseases that may cause wheezing and need to be differentiated from asthma include: (1) chronic sinusitis, (2) gastroesophageal reflux, (3) recurrent lower respiratory tract viral infections, (4) cystic fibrosis, (5) bronchopulmonary anomalies, (6) tuberculosis, (7) congenital developmental anomalies leading to intrathoracic airway narrowing, (8) foreign body aspiration, (9) primary ciliary dyskinesia, (10) immunodeficiency, and (11) congestive heart disease. The presence of symptoms in the neonatal period that interfere with development, symptoms associated with vomiting or the presence of pulmonary or cardiovascular signs suggest the need for further investigations. A useful way to establish the diagnosis of asthma in children ≤5 years of age is to inhale a short-acting β2 agonist and glucocorticoid for experimental treatment. Clinically significant improvement after treatment and deterioration after cessation of treatment support the diagnosis of asthma. Spirometry and other tests are recommended for older children and adults, but measurement of airway reactivity and airway inflammatory markers is more difficult, requires complex instrumentation, and is difficult to perform routinely. However, the use of peak velocimetry can be considered in children 4-5 years of age, although still with parental supervision and assistance.