Slavery

Slavery, as Paul LeVal wrote in 1861, was "one man owning another". Often, but not always, the enslaved were "different" than their oppressors. In LeVal's day, the basis of slavery was "skin colo[u]r or land of origin", but off Earth, (PROSE: Blood and Hope) some species would enslave other species. (TV: Planet of the Ood, Before the Flood)

Humans and slavery
The Sixth Doctor considered the slave trade to be the dark heart of England and one of Earth's worst abominations. (AUDIO: The Behemoth)

Daleks
Daleks forced many people into labour in Bedfordshire, (TV: The Dalek Invasion of Earth) as well as on Skaro itself. (TV: Destiny of the Daleks) Slavery was also present on the planet Nixyce VII. (AUDIO: The Traitor) Daleks also occasionally enslaved humans through physiological manipulation, such as when they created robomen or "humanoids". (TV: The Dalek Invasion of Earth, COMIC: The Humanoids) The Cult of Skaro once attempted to actually blend Dalek and human DNA to create a more reliable class of slave labour. (TV: Evolution of the Daleks) During the Last Great Time War. Daleks converted their enemies into Dalek Slaves. (COMIC: Four Doctors)

Other species enslaving humans
At least one Time Lord — — enslaved humans on a massive scale. Through fear alone, he got virtually the entire population of 21st century Earth to help him create what he termed "the New Time Lord Empire", a project that involved massive building works all around the planet. (TV: Last of the Time Lords)

The species behind the Company enslaved the members of the Vanir to work on the dangerous Terminus. (TV: Terminus)

On Ravolox, the robot Drathro enslaved five hundred humans residing there, whom he called "work units", in an underground facility. (TV: The Mysterious Planet)

During their invasion of Earth on Christmas Day 2006, the Sycorax planned to sell half of humanity into slavery. (TV: The Christmas Invasion)

In 1791, Joxer and Hempel came to London with the intention of kidnapping humans to sell as slaves, but were stopped by the Tenth Doctor, Rose Tyler and Chevalier D'Eon. (AUDIO: The Sword of the Chevalier)

Humans enslaving humans
At different points in their history, some humans enslaved other humans. In 2011, Luke Smith summed up humanity's history of slavery to his sister Sky Smith: "Humans used other humans as slaves for centuries. Every culture, the world over. Nobody ever challenged them until a few hundred years ago. And in some places it still goes on."

- Luke Smith

The Romans enslaved other people. Both Ian Chesterton and Barbara Wright were sold into slavery. (TV: The Romans) Centuries later, humans like Doland looked favourably upon the Roman Empire's use of slavery. (TV: Terror of the Vervoids) In Rome in 120, Vanessa Moretti was accused of being a runaway slave and was claimed by Balbus because she had no proof to refute this claim. (PROSE: The Stone Rose)

By the 15th century, it was against Spanish custom to engage in the slave trade. (AUDIO: Trouble in Paradise)

Slavery of Africans was institutionalised in the United States of America, enshrined in the Constitution of the United States. However, the nation was deeply divided on the issue, and there was a tenuous balance between those people — typically in the North — who wanted it abolished and those — often in the South — who saw slaveholding as a moral right. This tension finally exploded into the American Civil War. South Carolina, the first state to secede from the US, justified its action in a declaration which clearly cited US President-elect Abraham Lincoln's hostility to slavery as a main cause for secession. Since secession directly resulted in war, slavery was a central reason the war was fought. The eventual victory of Lincoln was something the Fifth Doctor, Peri Brown and Erimem were on hand to witness — although Erimem was sometimes unpleasantly mistaken as a slave because of her darker skin. (PROSE: Blood and Hope)

Likewise the English enslaved the Scottish during the mid-18th century. People like Solicitor Grey and Annabelle captain Trask routinely forced Scots from the Highlands to the West Indies and North America. They attempted to give it the veneer of legal propriety by having each Scot sign a contract, whereby their labour was exchanged for the "privilege" of ocean transport to their new "home", but these signatures were effectively coerced. Clearly, the Second Doctor and Ben Jackson saw it for the slavery it really was. The Doctor, in fact, deprived Grey of his contracts for one particular voyage, thereby removing even the veneer of propriety from Grey's activities. (TV: The Highlanders) Indeed, Grey himself privately confided to his co-conspirators that he knew very well he was sending the Highlanders into slavery, saying to Trask, "But to take these cattle fresh to the slave plantations — before their health has been sapped by His Majesty's prisons — that takes skill and preparation." (PROSE: The Highlanders)

Other parts of the world were involved in the slave trade, as well. The Fifth Doctor once told Turlough that he recalled seeing slave ships in the harbour at Rio de Janeiro in 1700. (AUDIO: Loups-Garoux)

While visiting London in 1599, Martha Jones was worried that she would "get carted off as a slave" because of her skin colour. (TV: The Shakespeare Code) Bill Potts voiced a similar worry while visiting 1814 as slavery was still "a thing" in that time. (TV: Thin Ice)

Humans enslaving other species
In the 21st century, there was a black market in alien slaves on Earth. John Harrison purchased a group of Skullion slaves on this market. (TV: The Man Who Never Was)

It was Doland's intent to turn the Vervoids into slave labour. Had he been able to deliver the sentient plants to Earth, he intended to use them as cheap replacements for robots. He didn't know, however, that the Vervoids themselves had the ability and desire to kill every bit of animal life they encountered. (TV: Terror of the Vervoids)

As Rose Tyler observed, humans enslaved the Ood in her far future. Even in remote outposts, some humans, like Danny Bartock, were charged with keeping order amongst the Ood by using a variety of technological devices, such as those which measured Basic, a scale for determining the level of Ood telepathic output. (TV: The Impossible Planet)

As Donna Noble would later discover, the humans of the 42nd century had industrialised and commercialised Ood slavery. By 4126, Ood Operations, the company in charge of the Ood, were having difficulties in the vast marketplace of the Second Great and Bountiful Human Empire. They began a new, aggressive advertising scheme that year which slashed prices, added "extra features" to the translator balls and generally attempted to "rebrand" Ood with Ood Operation's dealers. Donna and the Tenth Doctor were repulsed by this blatant commercialism. Together, they helped the Ood revolt and end human Ood trafficking. (TV: Planet of the Ood)

Tharils were traded by the humans because of their time sensitivity, used to pilot ships in the fourth dimension. Romana devoted herself to overthrowing this commerce. However, in a previous age, the roles had been inverted: humans had been enslaved by the Tharils. (TV: Warriors' Gate)

On Lobos, human zealots enslaved the native Loba. (PROSE: The Good Doctor)

Slavery between non-humans
Slavery was hardly a phenomenon exclusive to human culture. For instance, the Kaleds and Thals enslaved Mutos. (TV: Genesis of the Daleks)

Tivolians had an ongoing desire to be conquered and enslaved by others. (TV: The God Complex) When the Arcateenians freed them from the armies of the Fisher King, the Tivolians were distraught and irritated the Arcateenians so much that they chose to enslave the Tivolians themselves by the year 1980. (TV: Before the Flood)

After invading Gallifrey, Stor decided not to kill the Time Lords he had hostage, believing that "slavery [was] more efficient". (TV: The Invasion of Time)

Huducts enslaved Anubians by using mind-controlling torcs. After the Anubians were liberated by K9 Mark I, the Anubians turned on their former masters and enslaved them in the same way, even though K9 intended for them to reconcile in peace. The Anubians later developed space travel and enslaved other species. (TV: Curse of Anubis)

The Kleptons enslaved the Thains they abducted from their city. (COMIC: The Klepton Parasites)

The Voord on Kandalinga enslaved the native Fishmen to build dry land for them. (PROSE: The Fishmen of Kandalinga)

The Masters of Dorada enslaved the Doradans, forcing them to work in factories. (PROSE: The Dream Masters)

On a planet, the fair-haired people were the slaves of the black-haired people until they were freed by passing the tests of Trefus. (COMIC: The Tests of Trefus)

The Mentors from Thoros Beta took many Thoros Alphans from their homeworld for use as menial labour and experiments. Thanks to the intervention of Yrcanos and the Sixth Doctor, the control centre which allowed them to be enslaved was destroyed. (TV: Mindwarp)

The Janus Anah and Anahson came to the trap street in London to escape slavery. (TV: Face the Raven)

The Rocket Men owned hundreds of slaves. (AUDIO: Requiem for the Rocket Men)