Throughout many decades of travel in space and time, the Doctor has met thousands of people across the countless civilisations that have needed to be saved from legions of evil alien races. Of these thousands of individuals, a few lucky ones have been able to step aboard the TARDIS and embark on life-changing journeys with the Doctor, who has taken on a healthy number of friendly faces as companions.
The Doctor isn't much without his/her companions along for the journey. As well as aiding the Doctor in battling whatever monsters need defeating, the companions act as the audience's gateway into the lore of Doctor Who; they are essentially the life-blood of the show. Without them, the show would lose much of its emotion and charm (and besides, the Doctor needs someone to talk to). As Series 12 looms ever closer, it's good to have a chance to remember some familiar faces that were able to grace the halls (or is it roundels?) of the TARDIS.
Quick note: criteria for a companion here is that they must have journeyed in the TARDIS for more than one episode, meaning that some of the Doctor's friends that only lasted for a single episode won't qualify here. This list will mainly focus on companions from the revived era of the show, only because the classic era companions deserve a list of their own