Clone

A clone was (usually) a genetic copy of another biological entity. Clones could be produced to be exact replicas with short lifespans, as the Fourth Doctor did for himself and that of Leela at the Bi-Al Foundation, (TV: The Invisible Enemy) or they could be clones of individual cells which were grown into a useable organism, as Cassandra O'Brien did for herself following the destruction of the skin taken from the front of her body, on Platform One. (TV: New Earth)

Uses of cloning
A clone of Martha Jones was made by the Sontarans to infiltrate the United Nations Intelligence Taskforce. The clone had a mental link with the real Martha and could access her memories. If the link was broken, the clone would most likely not survive. The Sontarans used many clones to try to stop the Tenth Doctor from interfering with the conversion of the Earth's atmosphere and the subsequent invasion. (TV: The Sontaran Stratagem / The Poison Sky)

Using blood samples taken from the Sixth Doctor in 2004, the Forge operative known as Nimrod created a multitude of clones in order to discover the means of Time Lord regeneration. All these clones were of the Doctor's sixth incarnation, which suggests that Time Lord blood changed completely during regeneration. None of the clones could regenerate. They only had a one-tenth chance of surviving birth and would not live very long afterwards. Nimrod had abandoned this project by the time of the Seventh Doctor's visit to the Forge's base in Dartmoor in 2008. This was most probably because of Time Lord physiology, suggesting traditional cloning techniques would not work. It may also have had something to do with the Rassilon Imprimatur. (AUDIO: Project: Lazarus)

Using DNA from the Fourth Doctor, a clone of the Second Doctor was produced. He believed that he was the Doctor, and it even contained memories taken from the Doctor. However, he was much more sinister than the real Doctor and was in league with the enemies of the Doctor. The Fourth Doctor saved his life and he repaid him by helping return Hexford to its original place on Earth. Like the examples noted above, it was not believed to be able to last longer than a few months. (AUDIO: The Hexford Invasion, AUDIO: Survivors in Space)

Clones of the Three Who Rule were produced. (PROSE: Blood Harvest)

Clones of Galifrey were created during the the War against the Enemy and came to be known as Nine Gallifreys. (PROSE: The Shadows of Avalon)

Santa Claus used clones of himself to deliver presents on Christmas Eve. (PROSE: The Man Who (Nearly) Killed Christmas)

In June 2027, Ace told Henry Noone that Earth would have "new pandas" by 2040, suggesting that pandas had become extinct at some point in the early 21st century and were eventually brought back to life through the use of cloning. (AUDIO: A Death in the Family)

In the 22nd century, humans cloned Gangers from the Flesh as a workforce in industrial facilities. Some of these Gangers gained free will. Ganger technology continued in one form or another into the 52nd century. (TV: The Rebel Flesh / The Almost People)
 * For more information, see Ganger article.

In 5000, a short-lived clone replica of the Fourth Doctor and one of Leela were produced by Professor Marius and shrunken down to fight the Swarm inside the Doctor's brain. When the Swarm took control of Marius, he made a clone of Lowe to pursue them. These clones had lifespans of less than eleven minutes. (TV: The Invisible Enemy)

At some unknown point in the future, Professor Patrick Trethui cloned Vilgreth from the remains of a Titanthrope he had discovered. (AUDIO: Last of the Titans)

Races which use cloning
The Sontarans were a clone race, making each Sontaran almost identical to the other.