When the Klingons first made contact with the Federation in 2151 they appeared largely humanoid. The main differences centered around the heads, which bore a pattern of prominent ridges. By the mid 2200s the appearance of the Klingons had changed, making them almost totally Humanoid. By 2271 the species had changed again - most Klingons had acquired a single pronounced ridge of bone running laterally back across the skull; and finally by the 2290's all Klingons had returned to sporting a distinctive set of ridges occupying almost the whole frontal area of their heads. At first it was believed that some form of genetic engineering or viral mutation was responsible for these species-wide changes, but that such a widespread series of changes could so alter an advanced species against its will strains credibility a little. The Klingons themselves have consistently refused to provide any explanation for this transformation, simply stating that it is "a long story" that they "do not discuss with outsiders".

On average Klingons are larger and physically stronger than Humans. Many of their internal organs have redundant backups, allowing them to survive considerable injuries. Klingons are noted for having no tear ducts, and while most have red blood there are some races who's blood is distinctly pink. All Klingons suffer from certain allergies, most notably a strong reaction to small furry animals such as cats and tribbles.

By nature Klingons are a warrior race, and have been involved in many conquests throughout their history. Despite initial appearances of success their first contact with the Federation in 2151 was disastrous, leading to decades of conflict. Open warfare was was narrowly averted in 2266 by the intervention of the Organians. Over the next twenty five years the Federation and Klingon Empire made several attempts to coexist peacefully, with notable breakthroughs including the Korvat Negotiations in 2289 and the Khitomer Conference in 2293. Peace was finally achieved after the Narendra Incident, in which the crew of the Enterprise-C sacrificed themselves and their vessel to assist a Klingon colony under attack by Romulans. The peace held until 2372, when the Federations refusal to support the Klingons war against the Cardassians resulted in an attack on Deep Space Nine. The alliance was re-instated later in this year in order to present a united front against the Dominion threat. That alliance has continued to the present, and the two powers are currently engaged in joint operations against the Dominion.

In theory the Klingon Government consists of an Imperial structure with a High Council advising the Emperor. However, for centuries the title of Emperor was left vacant and the Chancellor of the High Council has ruled the Empire. This situation was modified in 2369 when Kahless apparently returned from the dead, although it was later learned that the returned Kahless was in fact a clone of the original created by Klingon scientists. Despite this, Chancellor Gowron allowed the cloned Kahless to take the imperial throne as a figurehead in order to promote unity among the Klingon people.

The Klingons fought alongside both the Federation and Romulans in the war against the Dominion, acquitting themselves with great bravery in that conflict. During the final stages of the war Gowron was killed in single combat by Lieutenant Commander Worf, who then appointed General Martok as Chancellor.