In 2009, an estimated 8.2 percent of Americans (9.6 percent of children and 7.7 percent of adults) had asthma, and the prevalence of asthma has increased substantially in recent years. In 2007, asthma accounted for 456,000 hospitalizations and more than 3,447 deaths. The goal of asthma treatment is to achieve asthma control, as evidenced by normal or near normal pulmonary function, maintenance of normal activity levels, and minimal need for shortacting beta2-agonist inhalers for "quick relief" of asthma symptoms. Persistent asthma treatment includes the use of long-term control medications (most commonly inhaled corticosteroids [ICS]) to reduce airway inflammation and quick-relief medications for acute exacerbations. While the benefits of asthma treatment generally outweigh the potential risks, these medications can be associated with adverse effects. Additionally, some asthma patients have concerns about asthma medications, and some patients would likely prefer to reduce their use of medication if alternative treatments were available. A number of nonpharmacologic methods for asthma management involve breathing retraining. Some of these, such as the Buteyko and Papworth methods, are predicated on the theory that asthma is related to hyperventilation. These treatments seek to reduce hyperventilation by encouraging shallow or slow nasal breathing, breath-holding at the end of expiration, and minimizing sighs and yawns and related breathing patterns that are characterized as "over-breathing." The idea behind these treatments is that hyperventilation leads to a reduction in blood and alveolar carbon dioxide (CO2), to which the airways respond by constricting to prevent further loss of CO2. The evidence supporting the hyperventilation theory of the pathophysiology of asthma is mixed. People with asthma do appear to have lower endtidal CO2 levels (i.e., blood levels of CO2 at the end of exhalation) than those without asthma. A reduction in end-tidal CO2 levels has been shown to increase airway resistance in people with asthma and a history of bronchial hyperresponsiveness to histamine, but not in matched controls without asthma. Further, airway resistance decreases when hypercapnia (high level of CO2 in the blood) is induced. Another study, however, found that longer breath-holding time was associated with a reduction in end-tidal CO2, which is counter to Buteyko's theory. The current review examines the effect of breathing retraining methods on asthma symptomatology, medication use, quality of life, and pulmonary function in both adults and children. We also examine adverse effects of these techniques. The analytic framework we developed to guide our review is shown in Figure A. The Key Questions for this review are as follows: In adults and children 5 years of age and older with asthma, does the use of breathing exercises and/or retraining techniques improve health outcomes, including symptoms (e.g., cough, wheezing, dyspnea); health-related quality of life (general and/or asthmaspecific); acute asthma exacerbations; and reduced use of quick-relief medications or reduced use of long-term control medications, when compared with usual care and/or other breathing techniques alone or in combination with other intervention strategies? In adults and children 5 years of age and older with asthma, does the use of breathing exercises and/or retraining techniques improve pulmonary function or other similar intermediate outcomes when compared with usual care and/or other breathing techniques alone or in combination with other intervention strategies? What is the nature and frequency of serious adverse effects of treatment with breathing exercises and/or retraining techniques, including increased frequency of acute asthma exacerbations?