High-quality, office-based spirometry provides diagnostic information as useful and reliable as testing performed in a pulmonary function laboratory. Spirometry may be used to monitor progression of lung disease and response to therapy. A stepwise approach to spirometry allows for ease and reliability of interpretation. Airway obstruction is suspected when there is a decreased forced expiratory volume in one second/forced vital capacity (FEV1/FVC) ratio, but there is no strong evidence to clearly define what constitutes a significant decrease in this ratio. A low FVC is defined as a value below the 5th percentile in adults or less than 80% of predicted in children and adolescents five to 18 years of age. The FEV1/FVC ratio and FVC are used together to identify obstructive defects and restrictive or mixed patterns. Obstructive defects should be assessed for reversibility, as indicated by an improvement of the FEV1 or FVC by at least 12% and 0.2 L in adults, or by more than 12% in children and adolescents five to 18 years of age after the administration of a short-acting bronchodilator. FEV1 is used to determine the severity of obstructive and restrictive disease, although the values were arbitrarily determined and are not based on evidence from patient outcomes. Bronchoprovocation testing may be used if spirometry results are normal and allergen- or exercise-induced asthma is suspected. For patients with an FEV1 less than 70% of predicted, a therapeutic trial of a short-acting bronchodilator may be tried instead of bronchoprovocation testing.