This issue of Clinics in Chest Medicine, edited by Dr. Denis O'Donnell and Dr. Alberto Neder, focuses on Clinical Respiratory Physiology. Articles include: ?The Pathophysiology of Obstructive Sleep Apnea; The Physiology of Mechanical Ventilation; Exercise Pathophysiology in Congestive Heart Failure; Control of Breathing; Breathing at Extremes; Exercise Pathophysiology in Interstitial Lung Disease;?Importance of Physiology in Clinical Decision-Making in the ICU;?Pulmonary Hypertension and Exercise;?Physiologic Effects of Oxygen Supplementation During Exercise in COPD; Benefits and Pitfalls of?DLCO measurements in Clinical Practice;?Cardio-pulmonary Interactions in COPD-CHF;?Exercise Physiology in COPD; Dyspnea of Unknown Origin: The Role of Exercise Testing; Assessment of Ventilatory Limitation During Exercise; Respiratory Muscle Assessment in Clinical Practice; Exertional Periodic Breathing in Heart Failure; and Strategies to Increase Physical Activity in Chronic Respiratory Diseases.