The Star Trek Expanded Universe is a generic term to describe an extrapolation of events which occur in the Star Trek Universe outside the scope of the television series and feature films. Information from the Star Trek Expanded Universe typically fills "holes" in the Star Trek story and timeline, with explanations of events which have never been adequately explained through live action productions. The term was first used in 1966 by writer D.C. Fontana to describe information put forth in the backstory of Doctor Leonard McCoy.

The publishing company Simon and Schuster is most directly responsible for contributing to the Star Trek Expanded Universe through a copyright on Pocket Books which has generated a large number of Star Trek novels over the past twenty five years. Information in the novels, while sometime contradictory, often serves to provide information to the Star Trek Expanded Universe.

The following events are discussed, explained, and portrayed in the Star Trek Expanded Universe.

• Leonard McCoy's Backstory: Leonard McCoy was married shortly before obtaining his Medical Degree. Starting a private practice, McCoy fathered a daughter before being divorced by his wife and losing custody of his child. Sometime after 2260, Leonard McCoy closed his medical practice and enlisted in Starfleet to escape the pain of losing his wife and daughter.

• Second Five Year Voyage of USS Enterprise: Following the events of Star Trek: The Motion Picture the Enterprise embarked on a second five year mission of exploration. James T. Kirk did not regain his rank of Rear Admiral but chose to remain a Captain to command the Enterprise.

• Chekov's Promotion History: Ensign Chekov was promoted to Lieutenant Junior Grade at the conclusion of the first five year voyage of the Enterprise. By 2271, he was a full Lieutenant. He obtained the rank of Lieutenant Commander during the second five year voyage of the Enterprise and, in 2276, was promoted to full Commander and assigned as the Executive Officer of the USS Reliant.

• Physical Appearance of the Klingons: The marked difference in the physical characteristics of the Klingons, as seen in Star Trek: The Original Series and in later feature films and television productions, was the result of a genetic engineering experiment which took place approximately between the year 2210 and 2270. In an attempt to emulate humans, and make infiltration in human regions of space easier, the Klingon Empire merged human DNA with a large segment of its military force. The result was the "Klingon-Human Fusions" who were the only Klingons permitted to have contact with the Federation. By 2271, the attempt at creating Klingon-Human Fusions had been abandoned and the Empire abolished the project. Certain high ranking Klingons (such as Kor, Koloth and Kang) were permitted to undergo procedures to remove the human DNA from their cell structures and revert to a pure Klingon appearance. To this day, the Klingon Empire considers the attempts at creating Klingon-Human Fusions to have been an embarrassment and it is "not discussed" with outsiders. As the "Expanded Universe" conjectures are not canonical, the producers of Star Trek are not bound by them; thus, in a two-part episode of Star Trek: Enterprise aired in 2005 ("Affliction " and "Divergence "), a somewhat different explanation for the change in Klingon physical appearance was given. Elements of the Expanded Universe conjecture (genetic engineering using human DNA) were retained, however it was revealed that the change was the result of a virus that resulted from these experiments, rather than from any voluntary choice on the part of the Klingons. Presumably a cure was found for the condition (which was passed from generation to generation) sometime prior to Star Trek: The Motion Picture.

• Physical Appearance of the Romulans: The ruling class Romulans of the 23rd century were referred to unofficially as Romulans of the "Smooth Brow". These Romulans were direct descendents of the original Vulcan dissidents that left Vulcan circa 1 AD (Earth Calendar). By the mid 24th century, the Smooth Brow Romulans had lost most of the power on Romulans to be replaced by standard Romulans who, through centuries of inbreeding and off-world marriages, had developed very pronounced forehead ridges. These Romulans occupied most positions of power in the 24th century government and military; however, the Romulans of the Smooth Brow could still be found in the public light. Commander Sela, a Romulan-Human cross, was often mistaken by fellow Romulans as a Smooth Brow. Ambassador Spock was also able to infiltrate Romulan society be posing as a Smooth Brow Romulan.

• The Riker-Troi Romance Backstory: although various hints have appeared in sundry Next Generation episodes, only the novel Imzadi actually told the whole story of how Riker and Troi originally met and fell in love - and of what came between them.