Over the years, the Doctor has had a lot of companions and not all of them have had the happiest of endings.
While some have been able to walk away on their own terms and live happily ever after, others have suffered the consequences of being associated with the Time Lord, and some have even died.
What is it about a companion exit that makes it so sad? We spend our time with these people and they see the universe through our eyes, so when the time comes for them to go, we feel like we're losing a friend and perhaps even a part of ourselves - because we can imagine ourselves in that situation and how we'd feel if it were us.
With companions Ace and Tegan recently returning for the Legend of the Sea Devils centenary special, and Yaz and Dan now both set to leave the show, it seems like a great time to look back on some of the saddest and most emotional companion exits in Doctor Who history.
10. Clara Oswald
Clara Oswald is the companion with the most departures. Her original ending came at the end of Season eight when she finally decided to leave the Doctor after the super sad "never trust a hug" scene, where both are hiding their real emotions.
Clara returned for the Christmas special, Last Christmas, which was set to wrap up her character arc with the final reveal that through all the dream sequences when the Doctor finally escapes and uncovers the truth, Clara is actually an old woman. This was changed again when Jenna decided to stay another series and another layer of dreams was added so that Clara could actually be fine.
Season nine saw Clara become more and more like the Doctor and played on the dangers of her becoming too much like her hero. It ultimately led to her death after she selflessly sacrificed herself to save a friend's life.
Clara's death scene in Face the Raven ended up hurting a lot because it proved how much the Doctor messed her up. Still, in two episodes time the Doctor would bring her back from the moment of death (which she would have to return to when she was ready) and allow her to live as an immortal. She even got her own TARDIS but the Doctor lost all memory of her.
The real tragedy was how a fitting, final sacrifice was cheapened and completely undone in one of New Who's most controversial moments that still divides fans to this day.