Roughly humanoid in appearance, Klingons typically sport long manes of luxuriant hair, and males, moustaches, and beards. Perhaps their most prominent external feature is their ridged forehead. These intricate, bony patterns, which are also visible on their feet, vary by family line and are an 'evolutionary' remnant of their prehistoric crustacean forms, when Klingons had a more extensive exoskeleton.

Most body functions incorporate multiple redundancies, such as redundant stomachs, lungs, livers, an eight-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 chain mail. The Klingon expected lifespan is at least 150 years

Klingon mating rituals involve limited domination and combative attitudes. Par'machpu' are chosen mates for dedicated recreational sexual congress, equivalent to fiancées among Terran humans. Klingon females reportedly search for their own partners, whom they deem worthy of copulation. Normally this has to be a male of great strength, valour and at least possess great courage. The mating process can be a very wild and sometimes violent affair. Klingon pregnancies run 30 weeks. The Process of giving birth can sometimes take days. Interbreeding is possible with Betazoids , humans , Romulans and Trill Thus they are all the same biological species. Klingon traits remain dominant over several generations.

The Klingon High Council feared that Starfleet was developing armies of Augments and that they would pose a serious threat to the Empire's existence. Even when they were told by the Vulcan High Command that the Augments were created without Starfleet's knowledge or consent, they remained suspicious and so decided to fight fire with fire. The Klingons gained access to the genetic material of the human Augments, and wanted to adapt this genetic engineering to augment their own species. The experiment did not work correctly; at first, subjects did gain increased strength and intelligence, but their nervous systems could not handle the strain and they died. One of the test subjects had a virulent flu, which — combined with the genetic changes wrought by the experiment — became a deadly, airborne plague that spread rampantly across the Empire, from world to world, causing the physical changes to change them into the human-looking Klingons of Kirk's day. Dr. Phlox of the Enterprise NX-01 formulated a cure for the virus, however not before millions of Klingons were physically altered. And owing to the genetic nature of the virus, these alterations were passed to succeeding generations of offspring. We believe that Section 31 was somehow involved in the transformation of the Klingons. Dr Phlox indicated that "someday" the physical alterations could be reversed. He was right the Klingon's found the cure to the physical alterations.