Airway obstruction is common during childhood and is often identified by acoustical signs. Vibration of airway walls in narrowed passages is a presumed mechanism of wheeze, a typical sound in patients with asthma and bronchiolitis. Other sources may include the oscillatory motion of airway secretions. Considering the significance of wheeze for epidemiological classifications, surprisingly little information exists about acoustical characteristics. Wheezing varies with age. Infants tend to have less musical sounds and complex repetitive sound waves compared with more regular sinusoidal patterns in older asthmatics. Children may not wheeze even when flow is significantly obstructed. However, normal lung sounds change during flow obstruction even without wheeze. Intensity at low frequencies (100--300 Hz) is typically reduced during obstruction while medium (300--600 Hz) and high frequency (600--1200 Hz) sounds---which are more affected by gas density and thus likely reflect turbulent flow---often increase. Compared with spirometry, measurements of respiratory sound can uncover flow obstruction in children without the need for forced breathing. Technical and theoretical questions remain, e.g., about sensors for noisy environments and about changes in thoracic sound transmission during bronchoconstriction. Nevertheless, acoustic measurements can provide a noninvasive diagnostic option with special usefulness in children.