Asthma is characterized by reversible lower airway obstruction. Pulmonary function tests (PFTs) are a useful way to objectively measure the degree, location, and reversibility of lung compromise in children who have or may have asthma. PFTs can be used to help confirm the diagnosis in children with straightforward or atypical presentations, estimate the severity of airway inflammation, and follow the response to management changes. In the initial evaluation of a child with straightforward asthma, PFTs can confirm the clinician's diagnosis as well as rule out problems that may present with similar clinical findings, such as muscle weakness, vocal cord dysfunction, and habit cough. The evaluation of children presenting with more atypical symptoms, such as chest pain, cough, or atypical exercise intolerance may be focused considerably by the use of PFTs. Reversibility of the obstruction as measured by increases or decrements in air flow or gas trapping provides an estimate of the degree of airway hyper-reactivity. The percentage improvement in air flow after bronchodilation increases in children with a large amount of airway inflammation, and decreases when the asthma is under control. Measurement of airway hyperreactivity can be helpful at the time of diagnosis, during times of seasonal exacerbations, and after aggressive anti-inflammatory therapy. This article reviews specific pulmonary function tests and examples of the use of PFTs in clinical situations that may be encountered in the management of children with asthma.