Asthma diagnosis is difficult in young children being mainly based on clinical signs and parents' history, which is sometimes difficult to obtain. Lung function testing may improve asthma diagnosis by objectively assessing its main features, airway obstruction, spontaneously reversible or after use of a bronchodilator drug, ventilation inhomogeneity during an acute bronchoconstriction and airway hyperresponsiveness. In young children that cannot cope with classical tests, it is important to use and develop simple, short lasting methods, made in spontaneous ventilation without active cooperation. Such techniques are a measurement of respiratory resistance by forced oscillations or the interrupter technique, of specific airway resistance by plethysmography and capnography. All these parameters are sensitive to the presence of an airway obstruction and to a bronchodilator or bronchoconstrictor agent, but their cutoff values in differentiating between asthmatic and healthy children as well as their specific indications in asthma management remain to be established.