Airway hyperresponsiveness (AHR) refers to an abnormal increase in airway responses, such as airflow limitation, to nonselective stimuli that are divided into direct (acting directly on the effector organs) and indirect (mediated via inflammatory or neuronal cells). The mechanisms of AHR in asthma have received much attention but remain uncertain. Well-standardized methods for measuring AHR represent important tools for the diagnosis and monitoring of asthma. The measurement of AHR is generally based on the dose-response curve that is obtained after exposure to a stimulus (usually inhalation of increasing doses of aerosolized mediator) followed by the assessment of lung function. Indirect challenges, as well as direct challenges, may provide useful information on the multiple pathways leading to AHR, towards improvements in the management of asthma.