The Sontarans are undoubtedly one of Doctor Who's most memorable monsters. Not as popular as the Daleks or the Cybermen, they are however one of the monsters who have returned to battle the Doctor through his regenerations.
The first Doctor to meet them was Jon Pertwee's Third Doctor in an adventure called The Time Warrior, in which a lone Sontaran is stranded on Earth during medieval times and is forced to kidnap scientists from Earths future in order to repair his crashed spaceship.
They next appeared in a two part adventure called The Sontaran Experiment in which another lone Sontaran Commander is conducting experiments on humans on the Earth of the future. This time the Doctor was played by Tom Baker.
They were next seen in the last few episodes of an adventure called The Invasion of Time when the Doctor's home planet was briefly subjugated by a race called the Vardans. Not everything is as it seems in this story, for although it looks as if the Doctor has turned traitor and is actually helping the invaders, he is actually setting the Vardans up in order to save Gallifrey. However what he doesn't realise is that the Sontarans are actually allies of the Vardans in their attempt to take control of Gallifrey! It was Tom Baker's Doctor again who had to thwart both races attempts to seize control of his home planet.
Their next adventure was several incarnations later when the Sixth Doctor discovers his old companion Jamie on the deserted space station Camera whilst trying to visit his old friend Dastari. The Sontarans have kidnapped the Second Doctor, Patrick Troughton, and Dastari is going to isolate the symbiotic nuclei of a Time Lord which allows them to control their time machines, the TARDIS. Appropriately enough this adventure is called The Two Doctors, and marks the return of both Patrick Troughton and Frazer Hines as the Doctor and Jamie to the series!
It wasn't until the show returned when the Sontarans next made an appearance in Doctor Who. David Tennant's Tenth Doctor met them in a two parter: The Sontaran Stratagem/The Poison Sky, when the Sontarans attempt to convert Earth into a breeding world for their clones, in which it is revealed that they are starting to lose their long war with the Rutans.
They have also cameoed in various other stories during the recent series, The End of Time part two, The Pandorica Opens, and more recently A Good Man Goes to War.
We have surely not seen the last of them! Perhaps they will return in the second half of Matt Smiths second series?