Asthma is a dynamic and complex inflammatory disease. Recent research suggests that it is a manifestation of a systemic disorder of the entire respiratory system including both upper and lower airways. The diagnosis of asthma can be made based on clinical history, physical findings, and pulmonary function tests such as spirometry. In children, spirometry may be difficult; therefore, diurnal changes in peak expiratory flow rate can be used instead to assist in the diagnosis of asthma. Increasing the use of objective pulmonary measures will help better identify and monitor treatment of lower respiratory inflammatory disease.