The defining feature of chronic obstructive pulmonary disease (COPD) is progressive deterioration in lung function. Measures of lung function are used to confirm the diagnosis, assess the severity of disease, and evaluate the efficacy of interventions. Forced expiratory volume in 1 second (FEV1), determined by spirometry, is the best known of these measures; however, it does not correlate well with dyspnea or exercise capacity, which are important targets for improvement in COPD management. Airflow obstruction in COPD often causes lung hyperinflation, which further inhibits the patient's ability to breathe. The degree of hyperinflation has been shown to correlate well with dyspnea and exercise capacity, but it is less convenient to measure than FEV1. This article briefly reviews the key lung function measurements used in monitoring patients with COPD. To illustrate how these measurements can be used to demonstrate the improvements in lung function elicited by effective bronchodilator therapy, the changes associated with the once-daily, long-acting bronchodilator tiotropium are presented.