Positive airway pressure (PAP) has been used since the 1930s to improve oxygenation, increase lung volumes and reduce venous return. More recently, PAP has been identified as an effective method of splinting airway during expiration, improving collateral ventilation, increasing response to inhaled bronchodilators, and aiding secretion clearance in patients with cystic fibrosis and chronic bronchitis. A range of devices, administration techniques, and evidence supporting their clinical use is explored, suggesting that PAP is equivalent to postural drainage in the clearance of secretions. PAP produced by threshold and fixed orifice resistors generate different characteristic flow, and airway and esophageal pressure patterns that may contribute to different physiologic effects. Further clinical studies are required to better understand the effects of these differences.