In the Doctor Who continuum, the Time Lord known as the Doctor often brings others along on his various adventures across time and space. Due to the Doctor's well-documented interest in humans, the vast majority of his companions have been human or descended from humans. However, he has been known to bring non-humans as well, even choosing machines as companions on occasion, such as K-9, who was not only a long-time companion of the Fourth Doctor, but eventually played key roles in two of the show's spin-offs.