The Trill are a humanoid species based on the planet Trill in the Alpha Quadrant, in which a small percentage of the population harbors a sentient lifeform known as a symbiont inside their bodies, a symbiotic relationship that plays a significant role in Trill culture and society. Such "joined" Trill have personalities that are a synthesis of the two beings, and can access the memories and skills of previous hosts.

The physical process of being joined was irreversible. Once joined, the host and symbiont were dependent on each other after a ninety-three-hour period in which the symbiont bound itself to its host's nervous system. If the symbiont was removed from the host, symbiont and host would die within hours, even if they were otherwise healthy, unless they were implanted into another host or with another symbiont.

On average, only three hundred symbionts were available for hosting each year and about a thousand Trills apply for joining. Because of this, candidate Trill hosts had to undergo an extensive selection process to show eligibility and fitness before they could be matched to a symbiont.

Personality: Unlike Vulcans or Klingons, Trill do not really possess a stereotypical personality. As with Humans, their personalities span the spectrum. Like most other Federation members, they are generally optimistic, open to new ideas, and willing to help those in need.

Physical Description: Trill have the same average height and weight as Humans. Their skin tones range from pale pink to olive, but no darker; their hair, on the other hand, tends to be brown, black, or other dark shades. They have two rows of dark brown spots running from their foreheads down their bodies to their heels. Each Trill’s spots are as distinctive to him as fingerprints are to a Human.

Example Names: Trill place the given name before the family name. Joined Trill substitute the symbiont’s name for the family name. First names: Curzon, Joran, Tobin, Nilani, Emony, Ezri, Jadzia, Lenara. Symbiont names: Dax, Belar, Pren, Tigan, Otner.