Objective measurement of airway function is a fundamental aspect of the diagnosis and management of asthma. Documenting reversible airway obstruction in a patient presenting with compatible symptoms establishes a diagnosis of asthma and helps to determine the appropriate level of therapy. Spirometry is the gold standard test of airway function. Bronchoprovocation testing and serial peak expiratory flow monitoring can also provide clinically useful measurements of the variable airflow obstruction characteristic of asthma. Home peak flow monitoring is often used as a component of longitudinal asthma management. Although it has many recognized limitations, peak flow monitoring has been shown to positively affect asthma outcomes, especially in patients with more severe disease.