Bronchial hyperreactivity has long been recognized as a hallmark of chronic asthma. Less is known about the prevalence and mechanisms of hyperreactivity in chronic obstructive pulmonary disease (COPD). Currently, it is unclear whether the role and mechanisms of hyperreactivity are similar in patients with asthma and COPD or whether the underlying pathophysiologic abnormalities are different for both diseases. The aim of this review is to present an overview of current knowledge of the mechanisms of bronchial hyperreactivity in asthma and COPD.