When the Klingons first made contact with the Federation they appeared almost totally humanoid. By 2271 most Klingons had acquired a single pronounced ridge of bone running laterally back across the skull; and by the 2290's all Klingons had 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 this species-wide change, but historical evidence indicates that the Klingons had this appearence throughout most of their history, becoming humanoid sometime prior to 2218 and then gradually reverting back to their normal appearence. The Klingons 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 whos 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. Their first contact with the Federation in 2218 was disasterous, and led to nearly a century of hostilities. 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 Khittomer 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.