Wheezing in preschool children is very common, with a wide differential diagnosis. It is essential to be sure of the exact sound that parents are describing; the term 'wheeze' is often applied to non-specific sound.Approximately one-third of children are diagnosed with wheeze in the first 3 years of life, making wheeze one of the commonest respiratory symptoms.The differential diagnosis of wheeze is wide, and different management strategies are needed depending on the underlying phenotype. The word 'wheeze' is used to describe many different sounds. That can be heard by both clincians and parent. Even if true wheeze is heard, this should not be automatically assumed to be due to bronchospasm. Airway narrowing by mucus will produce true wheeze but does not respond to bronchodilators. Similarly, airway malacia, either related to intrinsic airway wall defects or loss of alveolar tethering points, are also causes of bronchodilatorunresponsive wheeze; indeed, bronchodilators, by reducing airway smooth muscle tone, may actually worsen airway narrowing.