Everything has it's time, and everything dies. That's certainly true when it comes to Doctor Who characters. Ths show goes through an awful lot of supporting cast due to the Doctor travelling to a different location every week, and a lot of those supporting characters are killed. Often this is to demonstrate the danger that the main characters face, or to set up some sense of mystery, dramatic irony or catharsis. Whatever the reasoning behind them, these deaths have led to a number of memorable scenes.
They've also led to the writers getting reputations for being character-killers (we're looking at you, Moffat! Though the others aren't far behind... Russell T Davies death rate is suprisingly high).
Key deaths within the series haven't just been restricted to supporting characters, either. There wouldn't be much tension if the heroes always survived, and sometimes even main or recurring characters don't make it out of an episode alive. Some do manage to make it out of an episode, but still die over the course of the adventure. An awful lot of villains don't tend to make it either, which isn't always a bad thing as it means our heroes win, but these deaths can be just as painful. Some villains you just can't hate.
There won't be any regenerations on this list, as it's debateable whether this counts as a death or is merely a character changing into a new form. With that out of the way, let's follow the Grim Reaper back through time.