Romulans are a fictional alien species in the Star Trek universe that are relate to the Vulcans and are at war or uneasy truce with the humans of Earth and the United Federation of Planets throughout most of the Star Trek series and films. They are characterized as passionate, cunning, and opportunistic, and were usually cast as villains or adversaries in the various series. They are the dominant race of the Romulan Star Empire, one of the larger empires in the Beta Quadrant of the Milky Way galaxy.