The Library Computer Access and Retrieval System (LCARS or simply library computer for short) is the name of the computer operating system employed by the United Federation of Planets, used aboard Starfleet vessels. It is accessible by both voice and keypad commands.

LCARS uses sophisticated artificial intelligence routines to understand and execute vocal natural language commands. Starfleet personnel also frequently interface with LCARS with PADDs, tricorders and control panels.

In the 24th century, graphical controls housed underneath touch-sensitive clear panels allow LCARS panels to be quickly reconfigured by users to suit the task at hand, including a tactilE interface for visually-impaired officers (VOY: "Year of Hell"). This enables even complicated tasks to be executed with just a few button presses. LCARS also controls the retrieval and storage of files in the databanks housed within the ship's computer cores.

Depending on the starship, the LCARS design, especially the color scheme, differs. These colors can range from a yellow/white (found on the USS Enterprise-D) to blue/white scheme (found on the USS Enterprise-E).

In an alternate timeline in the 25th century, LCARS is further upgraded with three-dimensional control interfaces. (DS9: "The Visitor")

By the 29th century the LCARS system is, at least partially, replaced by the TCARS system.

• LCARS are seen in almost every episode of The Next Generation, Deep Space Nine, and Voyager. Whether the interfaces seen in The Original Series and Enterprise are LCARS are debatable, since the designs are quite different and can vary, unlike those in TNG, DS9, and VOY, where they all retain the same look.

• It is debatable whether or not the creator of the LCARS was Dr. Michael Okuda, an in-universe character regularly seen on background graphics.

• The date of the invention of the LCARS is not known although the modern LCARS was first installed on board the Enterprise-D.

• The full name "Library Computer Access and Retrieval System" could be seen in "The Price", for example.

• The sickbay used in Star Trek V: The Final Frontier is a minor redress of the Star Trek: The Next Generation sickbay, and the LCARS panels on the wall are barely altered, so this could be the first LCARS appearance. However, they do not appear in Star Trek VI: The Undiscovered Country, or the 23rd century portion of Generations.

• Viewers may notice that in the first two seasons of The Next Generation, large black rectangles are clearly visible on the LCARS displays on the bridge (and sometimes in main engineering). This was a result of the studio lights reflecting off the displays, which director of photography Edward R. Brown tried to solve by sticking cardboard onto them. When Brown was replaced by Marvin V. Rush for the third season, a number of changes in filming (including better film stock and a smaller number of lights) allowed the LCARS displays to be seen properly.