Breathing exercises and breathing retraining are often used in the management of asthma. One specific form of breathing therapy, known as the Buteyko breathing technique (BBT) has received considerable attention, but there is a paucity of rigorous research evidence to support its recommendation for asthma patients. There are only four published clinical trials and two conference abstracts evaluating BBT. Although all have reported improvements in one or more outcome measures, results have not been consistent. This article provides the background to the BBT, reviews the available evidence for its use and examines the physiological hypothesis claimed to underpin it. In common with other therapies, BBT is not a standardised treatment modality. The BBT 'package' is complex, as it also includes advice and education about medication use, nutrition and exercise, and general relaxation. This makes it difficult, and possibly inappropriate, to attempt to tease out a single mechanism. Buteyko's theory relating to carbon dioxide levels and airway calibre is an attractive one, and has some basis in evidence from experimental studies. However, it is not known whether altering breathing patterns can raise carbon dioxide levels significantly, and there is currently insufficient evidence to confirm that this is the mechanism behind any effect that BBT may exert. Further research is necessary to establish unequivocally whether BBT is effective, and if so, how it may work.