A self-contained spacesuit system that provides an astronaut with oxygen and removes carbon dioxide. It is worn on the back, and the oxygen goes into the hard upper torso and then the back of the helmet, so the visor does not fog up. As it passes down the astronaut's face, the air removes carbon dioxide and humidity, continuing down the body before a fan pulls it back into the PLSS where a contaminant control cartridge filters out the carbon dioxide. The oxygen then passes through a carbon dioxide sensor before returning to the astronaut. A secondary oxygen pack (SOP) is attached at the bottom of the PLSS to activate if the first system fails.

The PLLS was developed in 1976 by the Hamilton Standard Division of United Technologies. PLLS originally was the abbreviation for portable life-support system (used by Apollo astronauts) until a secondary oxygen system was added.