Lieutenant Reginald Endicott "Reg" Barclay III, portrayed by Dwight Schultz, is a fictional character from both Star Trek: The Next Generation and Star Trek: Voyager, in the latter of which he plays a vital role in re-establishing regular contact between the starship and Starfleet. The character also appears in the 1996 feature film Star Trek: First Contact.

Barclay was an occasional character in The Next Generation as an engineer, often being used as comic relief. His addiction to the holodeck is first seen in the episode "Hollow Pursuits", in which he creates holographic reproductions of the ship's bridge officers, who are completely responsive to Barclay's every whim. Being totally unlike their ship-board counterparts, they serve to bolster his self-esteem. With encouragement from Chief Engineer Geordi La Forge, however, Barclay redeems himself and helps to uncover the cause of a critical multi-system failure on the Enterprise in time to avert the ship's destruction.  In the episode "The Nth Degree", Barclay's brain is mutated by an ancient race from the center of the Milky Way galaxy, the Cytherians, radically increasing his intellect. Under this influence, Barclay seizes control of the Enterprise and brings the ship into contact with the Cytherians. Barclay explains to Picard that they only desire an exchange of information with the Federation. After the exchange, the Cytherians return the Enterprise to Federation space, restoring Barclay to normal and leaving him with only the memory of his actions and an enhanced ability at chess. This episode also sees Barclay try his hand at acting, performing the title role in Dr. Crusher's production of Cyrano de Bergerac. While his unaided performance was poor, his enhanced performance riveted the onlookers and greatly impressed Dr. Crusher.  In the episode "Realm of Fear", Barclay deals with his transporter phobia when assigned to an away team. Though he becomes capable of allowing himself to be transported, he believes that he sees large worm-like creatures while in transit. When no evidence of a problem is found, Barclay believes himself to be going mad. However, he later discovers that these are actually human survivors trapped inside the transporter beam. With Barclay's help, these people are successfully rescued.  In "Ship in a Bottle", Barclay unwittingly revives a sentient holo-simulation of Sherlock Holmes' arch-foe, Professor James Moriarty (previously seen in "Elementary, Dear Data") while performing holodeck maintenance. This results in Picard, Data, and Barclay unknowingly becoming trapped in a Moriarty-created simulation of the Enterprise, and forced to research a method of making holo-simulations "real" outside of the holodeck through Moriarty's manipulations. When the ruse is discovered, Moriarty and a holographic companion are tricked into a simulation within the simulation, and stored in a computer where they will continue to exist, believing that they have gained freedom from the holodeck.  In "Genesis", Barclay's T-cells have a bizarre reaction to a medicine, creating an airborne virus that causes Barclay to de-evolve into a spider-like ancestral form, and causes its other victims to revert to their various evolutionary forms, after multiple dormant introns in his body are accidentally activated. Upon recovery, the disease is named after him as he was the first to have had contracted it: Barclay's Protomorphosis Syndrome.

What else did Barclay do?
Barclay's brain is mutated by an ancient race from the center of the Milky Way galaxy,