Over the years, the Doctor has survived a hell of a lot of dangerous encounters. In almost every single episode, the character stares death directly in the face and lives to tell the tale, but even though their Time Lord biology makes them a tough cookie to defeat (extra durability, two hearts, and so on) the Doctor isn't invincible - far from it.
In fact, there are plenty of situations - some theoretical, and some that are actually seen in the show - in which the Doctor would be outright killed, meaning that they're unable to heal their injuries and regenerate into a new body.
Of course, the writers aren't going to kill the main character off, so it's incredibly rare for us to see these deadly methods in use against the Doctor. But from time to time, we're shown that regenerating sometimes isn't an option, and that there are ways to kill the Doctor permanently, difficult though that may be.
So, for all you Time Lord bounty hunters out there, here are a few ideas to make your killing sprees a lot more efficient...