Daleks First Appearance: "The Daleks," Season 1, 1963 Why They're Important: Well, for one thing, they're the most long-lived of the Doctor Who monsters -- and also the most popular, appearing in every season of the revived show since 2005. They're also an integral part of the larger Who mythology, having been named as the enemies of the Time Lords during the Great Time War which eventually led to the eradication of the Doctor's race. Not bad for a pun of shrill-voiced pepper pots.

Autons First Appearance: "Spearhead from Space," Season 7, 1970 Why They're Important: The Autons -- life-size plastic dummies animated by an alien presence called the Nestene Consciousness -- occupy a couple of particular places in Who history, being the first monsters to appear in a color episode of the series (Yes, the first six years of the show were in black and white), and also the first monsters to appear in the revived version of the show in 2005. As such, they were also the first monsters to encounter the Third and Ninth Doctors, respectively.

Slitheen First Appearance: "Aliens of London," Revived series Season 1, 2005 Why They're Important: The Doctor Who Monster equivalent of Mean Girls' "Fetch," the Slitheen were monsters that creator and then-current Who showrunner Russell T. Davies just couldn't let go. They were far more comedic than threatening -- as if they didn't look ridiculous enough, they also had problems with intestinal gas -- and, after a couple of unsuccessful appearances as villains on Who, quickly became recast as comic relief or unscary threats on kid-centric spin-off The Sarah Jane Adventures. In other words, consider them to Doctor Who what the Ferengi were to Star Trek.

Ood First Appearance: "The Impossible Planet," Revived series Season 2 (2006) Why They're Important: The Ood are a truly strange case of science-fiction metaphor meeting Hollywood cliche. When we initially meet them, they're an enslaved race who, we're told, have to be kept in servitude for their own good. Of course, this doesn't turn out to be the case, and eventually freed, they evolve into a race of wise, knowing beings who offer guidance and spirituality to others. Whether or not swapping the stereotype of the black slave with the stereotype of the Magical Negro was intentional remains unclear.

Cybermen First Appearance: "The Tenth Planet," Season 4 (1966) Why They're Important: Arguably the second-best known of the Doctor Who monsters, the Cybermen represent a particular brand of body (and mental) horror -- what happens to humanity when it gets a little too obsessed with upgrading itself with the use of technology. Part Frankenstein myth, part commentary on group-think and the herd mentality, the Cybermen could be the most chilling of all of the Who monsters -- if only you could ignore their radiator heads.

Weeping Angels First Appearance: "Blink," Revived series Season 3 (2007) Why They're Important: Who has a history of making monsters out of everyday objects, as the Autons demonstrate. The Weeping Angels may be the zenith of that particular trend, however -- alien statues that can move only when you're not looking at them, and who are slowly coming to kill you (or, perhaps, transform you into one of them, or maybe throw you back in time to eat your misery. It gets less clear the more often they appear). Their first episode, "Blink," is one of Who's finest moments in the entire 50-year run.

Sontarans First Appearance: "The Time Warrior," Season 11 (1973) Why They're Important: Doctor Who is, in many ways, an essentially pacifist show, and one of the places where that may be most obvious is with the Sontarans -- a militaristic race of cloned soldiers who want only war and, more importantly, conquest of others. Despite that, the Sontarans are ridiculous, especially in the revived series' take on them, where their machismo is played for laughs and reinforced with a lack of critical thinking -- or even much intelligence at all, outside of strategic planning. Only Doctor Who would have such glee in satirizing militarized cultures.

Silurians First Appearance: "Doctor Who and the Silurians," Season 7 (1970) Why They're Important: The original "owners" of Earth, the Silurians -- also known as the Sea Devils for reasons too oblique to explain here -- are that most welcome of things: Villains who might, actually, have the moral superiority in their conflict with humanity. Since their reappearance in the show's revived incarnation, they've managed to both be partially responsible for the (temporary) death of one of the Doctor's companions and also offered up the fabulous Madame Vastra, Victorian detective and the show's quasi-inspiration for Sherlock Holmes.

Ice Warriors First Appearance: "The Ice Warriors," Season 5 (1967) Why They're Important: There's something wonderfully classic about the Ice Warriors' core concept: They're Martians who are invading Earth because they've managed to destroy their own planet and would like a replacement. Although the Warriors have suffered in comparison to other Who monsters -- in many ways, they're like a combination of the Daleks and the Cybermen in terms of motivation -- they reappeared in the most recent season of the revised show, suggesting their particular cold war might be about to heat up again in future episodes.