Airway hyperresponsiveness is the hallmark of asthma. Tests of airway responsiveness are therefore most commonly used to confirm or exclude a diagnosis of asthma. The airway calibre of normal, healthy individuals responds on exposure to an appropriate stimulus. A distinguishing feature of asthmatic airways is hyperresponsiveness, meaning that the bronchoconstrictor response may occur at a far lower threshold than would be seen in a healthy individual. Tests of airway responsiveness tend to be time consuming in comparison to other pulmonary function tests, due to the delay required for a specific intervention to take effect. Good subject compliance with reproducible baseline results is requisite to performing responsiveness testing. Reversibility testing involves the evaluation of airway function before and after the administration of a bronchodilator agent. Pharmacological challenge tests involve administration of incremental dosages of a bronchoconstrictor substance and assessing changes in airway function after each incremental dose. Safety precautions are required, should severe airway constriction occur.