U.N.I.T. are a fictional military force that was created to help Doctor 3 defeat threats on Earth. U.N.I.T. are not eclusive to the Third Doctor. They have appeared with Doctor 10 in the long episode Doctor Who - planet of the Dead. U.N.I.T.`s full name is United Nations Intelligence Taskforce and was originally led by Briigadier Lethbridge-Stewart. They`ll return to help the Doctor in future adventures.