Obstructive airway disorders and bronchial hyperreactivity are part of the definition of asthma. Lung function tests (LFT) are therefore essential in the diagnosis and follow-up of patients with asthma. Maximum forced expiratory volume in one second (FEV1) is the essential functional indicator of severity and reversibility, according to the Global Initiative for Asthma (Gina). FEV1 has some limitations: it is poorly correlated with dyspnea, symptom control, and lung distention. In severe asthma, FEV1 is the only functional indicator included in the diagnostic criteria. It is one minor criterion among 2 major and 7 minor criteria. Other indicators measured during standard LFT and new functional indicators not yet sufficiently validated should provide important supplementary information to help to characterize the different clinical phenotypes and the pathophysiologic mechanisms involved in refractory asthma.