Background: Water-based exercise has long been used as a therapeutic modality for the treatment of a wide variety of health conditions. However, head out of water immersion (HOWI) was previously thought to be detrimental to the respiratory system and was not recommended for people with chronic obstructive pulmonary disease (COPD). Recent research has provided evidence to the contrary. In fact, HOWI and physical exercise training in water may improve exercise capacity and health-related quality of life in people with COPD. Objective: To review the physiological effects of HOWI and the safety and efficacy of water-based exercise for people with COPD. Major findings: HOWI and exercise in water can be performed safely without any adverse events and with maintenance of oxygen saturation even in those with severe COPD. Evidence is beginning to emerge on the beneficial effects of water-based exercise training on exercise capacity and quality of life in COPD although further high quality research is required. Conclusion: Water-based exercise is safe and provides a promising treatment approach which may be used to assist in the management of COPD.Keywords: Water-based exercise training, COPD, Physiology of water immersion Correspondence to: R J McNamara,