Positive end-expiratory pressure (PEEP) is one of the most frequently discussed topics in critical care medicine. However, alveolar pressure can remain positive throughout expiration without PEEP set by the ventilator whenever the time available to breathe out is shorter than the time required to decompress the lungs to the elastic equilibrium volume of the total respiratory system (Vr). The end-expiratory elastic recoil (Pel,rs) due to incomplete expiration has been termed auto PEEP, occult PEEP [1], inadvertent PEEP [2], endogenous PEEP, internal PEEP and intrinsic PEEP [3, 4] owing to its similarity and contrast with PEEP set by the ventilator. The purpose of this article is to provide a comprehensive review of the studies on this interesting aspect of critical care medicine, from the underlying physiological mechanism(s) to the clinical and therapeutic implications, through measurement and monitoring in the intensive care setting.