Chronic obstructive airways disease (COPD) and asthma are major causes of morbidity around the world. This review examines the evidence on the effectiveness of tiotropium as a treatment for COPD and asthma. It discusses the role of acetylcholine in airway physiology and the effects of muscarinic antagonism on airways smooth muscle, mucous secretion and inflammation. In COPD, tiotropium increases forced expiratory volume in 1 second (FEV 1) and reduces hyperinflation; it improves breathlessness, exercise capacity and health status, and it reduces exacerbation rates. Tiotropium also reduces the rate of decline in FEV 1 over 4 years in patients with an FEV 1 over 50% predicted. In asthma, when added to inhaled steroids alone or in combination with long-acting beta agonists, tiotropium improves lung function, improves asthma control and reduces exacerbations. These effects have been seen in children, adolescents and adults with asthma. Tiotropium is well tolerated, with a low incidence of adverse events.