Adversaries

When Sydney Newman commissioned the series, he specifically did not want to perpetuate the cliché of the "bug-eyed monster" of science fiction. However, monsters were popular with audiences and so became a staple of Doctor Who almost from the beginning.
With the show's 2005 revival, executive producer Russell T Davies stated his intention to reintroduce classic icons of Doctor Who one step at a time: the Autons with the Nestene Consciousness and Daleks in series 1, Cybermen in series 2, the Macra and the Master in series 3, the Sontarans and Davros in series 4, and the Time Lords (Rassilon) in the 2009–10 Specials. Davies' successor, Steven Moffat, has continued the trend by reviving the Silurians in series 5, Cybermats in series 6, the Great Intelligence and the Ice Warriors in Series 7, and Zygons in the 50th Anniversary Special. Since its 2005 return, the series has also introduced new recurring aliens: Slitheen (Raxacoricofallapatorian), Ood, Judoon, Weeping Angels and the Silence.
Besides infrequent appearances by the Ice Warriors, Ogrons, the Rani, and Black Guardian, several adversaries have become particularly iconic:
Daleks

Dalek

The Dalek race, which first appeared in the show's second serial in 1963, are Doctor Who's oldest villains. The Daleks were Kaleds from the planet Skaro, mutated by the scientist Davros and housed in tank-like mechanical armour shells for mobility. The actual creatures resemble octopuses with large, pronounced brains. Their armour shells contain a single eye-stalk to allow them vision, a sink-plunger-like device that serves the purpose of a hand, and a directed-energy weapon. Their main weakness is their eyestalk; most attacks on them, including those from guns and baseball bats, will blind them, making them go mad. Their chief role in the plot of the series, as they frequently remark in their instantly recognisable metallic voices, is to "exterminate" all non-Dalek beings, even attacking the Time Lords in the Time War, which was not shown until the 50th Anniversary celebrating the show, where some snippets of the Time War are shown. The Daleks' most recent appearance was in the 2013 episode "The Time of the Doctor". They continue to be a recurring 'monster' within the Doctor Who franchise. Davros himself has also been a recurring figure since his debut in Genesis of the Daleks, although played by several different actors.
The Daleks were created by writer Terry Nation (who intended them to be an allegory of the Nazis) and BBC designer Raymond Cusick. The Daleks' début in the programme's second serial, The Daleks (1963–64), made both the Daleks and Doctor Who very popular. A Dalek appeared on a postage stamp celebrating British popular culture in 1999, photographed by Lord Snowdon. In the new series, Daleks come in a range of colours; the colour of a Dalek denotes its role within the species.
In the 2012 episode "Asylum of the Daleks", every generation of the Dalek species made an appearance.

Cybermen

Cybermen were originally a wholly organic species of humanoids originating on Earth's twin planet Mondas that began to implant more and more artificial parts into their bodies. This led to the race becoming coldly logical and calculating cyborgs, with emotions usually only shown when naked aggression was called for. With the demise of Mondas, they acquired Telos as their new home planet. They continue to be a recurring 'monster' within the Doctor Who franchise.
The 2006 series introduced a totally new variation of Cybermen. These Cybus Cybermen were created in a parallel universe by the mad inventor John Lumic; he was attempting to preserve the life of a human by transplanting their brains into powerful metal bodies, sending them orders using a mobile phone network and inhibiting their emotions with an electronic chip. In November 2012, Neil Gaiman confirmed that the Cybermen would feature in an upcoming series 7 episode he has written. This episode, "Nightmare in Silver", was broadcast in 2013.

The Master

The Master is the Doctor's archenemy, a renegade Time Lord who desires to rule the universe. Conceived as "Professor Moriarty to the Doctor's Sherlock Holmes", the character first appeared in 1971. As with the Doctor, the role has been portrayed by several actors, since the Master is a Time Lord as well and able to regenerate; the first of these actors was Roger Delgado, who continued in the role until his death in 1973. The Master was briefly played by Peter Pratt and Geoffrey Beevers until Anthony Ainley took over and continued to play the character until Doctor Who's hiatus in 1989. The Master returned in the 1996 television movie of Doctor Who, and was played by American actor Eric Roberts.
The Master has appeared in the revived series, portrayed for one episode by Derek Jacobi before the character regenerated, and otherwise John Simm since then.