Klingons (Klingon: tlhIngan) are a warrior race in the fictional Star Trek universe. They were recurring antagonists in Star Trek: The Original Series (TOS) and were later featured in all five spin-off series, eventually becoming uneasy allies of the United Federation of Planets.
Created by screenwriter Gene Coon, Klingons were introduced in the episode "Errand of Mercy" (1967). They were named for Lieutenant Wilbur Clingan, who served with Star Trek creator Gene Roddenberry in the Los Angeles Police Department.
The series has made several references to Klingon anatomy. Most Klingon body functions incorporate multiple redundancies, such as redundant stomachs, lungs, livers, an eight-chambered heart (although the Star Trek Medical Reference Manual shows a three-chambered heart), and twenty-three ribs.  This characteristic, known as "brak'lul",  makes Klingons incredibly resilient. Klingon ribs are arranged in a latticework; the structure might be compared to chainmail. Klingon teeth are typically serrated, with multiple edges and ridges.
Mr. Spock once said Klingons lack tear ducts, although Klingon myth states that Kahless once filled the ocean with his tears. The Klingon expected lifespan is at least 150 years (in one movie novelization it was stated that Klingons were "old" at 45 and aged much quicker than humans). However, it is typical that male Klingons die young while in battle and not of natural causes.
Star Trek VI: The Undiscovered Country depicts Klingons having violet blood, but all other depictions of Klingon blood have been red like human blood. Director Nicholas Meyer explained in the DVD commentary that the reason for the blood being colored pink was because in the original filming it was meant to be colored red but censors argued that doing so made the film too gory and would necessitate an "R" rating. Changing the color of the blood to pink satisfied the censors, as it no longer resembled human blood, and the film was rated "PG." Meyer saw no need to explain the color of the blood since Klingons were not human and there was no reason to assume their blood would be the same color.
Broken Bow seems to indicate that Klingon erythrocytes contain DNA, unlike their human counterparts.
Qo'noS is the Klingon homeworld, pronounced as Kronos. The Klingon High Council, governing body of the Klingon Empire, is situated in the First City on Qo'noS. Early Star Trek literature referred to Qo'noS as "Klinzhai", but the TNG episode Heart of Glory called the planet "Kling". It was the film Star Trek VI which officially established the name as Qo'noS. Qo'noS is depicted as green in color when viewed from orbital space. It includes a lone huge land mass with a vast ocean, a severely tilted axis that causes wild seasonal changes, a turbulent atmosphere and extremes of both warm and frigid weather. Around the 14th century, Qo'noS was invaded by the Hur'q. The planet was first visited by Humans in 2151. The planet had a moon named Praxis until 2293 - when the moon was destroyed in an explosion (Star Trek VI: The Undiscovered Country). Praxis had been a key energy production facility for the Klingon Empire prior to its destruction. When it exploded, it caused severe ecological damage to Qo'noS. As a result, the Klingons reached out peacefully to the other races of the galaxy to help deal with this crisis - which would eventually result in a formal alliance with the United Federation of Planets.
The Klingon language (tlhIngan Hol in Klingon) is the constructed language spoken by Klingons in the fictional Star Trek universe. Deliberately designed by Marc Okrand to be "alien", it contains many peculiarities, such as Object Verb Subject (OVS) word order. The basic sound (along with a few words) was first devised by actor James Doohan ("Scotty") for Star Trek: The Motion Picture. That film marked the first time the language had been heard on screen; in all previous appearances, Klingons spoke in English. Klingon was subsequently developed by Okrand into a fully-fledged language.