The evidence base for diaphragmatic breathing (DB) as an adjunctive treatment modality for persons with COPD is questionable. This article reviews the literature regarding the efficacy of DB in persons with chronic obstructive pulmonary disease (COPD), and reports on the beneficial and detrimental effects of DB in persons with COPD. Diaphragmatic breathing has been described as breathing predominantly with the diaphragm while minimizing the action of accessory muscles that may assist with inspiration. No single or combined patient characteristic has been identified consistently to help predict which person with COPD may benefit from DB. However, it has been suggested that persons with moderate to severe COPD and marked hyperinflation of the lungs without adequate diaphragmatic movement and increase in tidal volume during DB may be poor candidates for instruction in DB. Conversely, persons with COPD who have elevated respiratory rates, low tidal volumes that increase during DB, and abnormal arterial blood gases with adequate diaphragmatic movement may benefit from DB. Identification of an abdominal paradoxical breathing pattern and worsening dyspnea and fatigue during or after DB are criteria to modify or terminate DB. Persons with COPD demonstrating an abdominal paradox during DB may benefit from a more upright body position or trunk flexion. Several methods to examine diaphragmatic movement and the potential for success with DB will be discussed. Future research is needed to better identify which patients may benefit from DB.