Slavery in the ancient world and in Rome was vital to both the economy and even the social fabric of the society. While it was commonplace throughout the Mediterranean region, and the Hellenistic regions in the east, it was not nearly so vital to others as it was to the dominance of Rome. As the Romans consolidated their hegemony of Italy and Sicily followed by the systematic conquest of western Europe, countless millions of slaves were transported to Rome the Italian countryside and Latin colonies all over Europe.
Though slavery was prevalent in households throughout the city itself, it was on the farms and plantations where it had its greatest effect. The Roman conquests of Carthage, Macedonia and Greece in the 3rd and 2nd centuries BC altered what was once a luxury and privilege for the ruling elite into the predominant factor driving both social and economic policies for the Republic as a whole. The mass influx of slaves during this time period first was a sign of great wealth and power, but later destabilized an already fragile Roman class system. Farms originally run by small business families throughout Italy were soon gobbled up and replaced by enormous slave run plantations owned by the aristocratic elite. Cheap slave labor replaced work for the average citizen and the rolls of the unemployed masses grew to epidemic proportions. These issues had a great destabilizing effect on the social system which had a direct role in the demise of the Republic. As the rift between Senatorial elite (optimates) and social reformers (populares) grew, the use of the unemployed, landless, yet citizen mobs were an overwhelming ploy grinding away at the ability of the Senate to govern. Though there are many factors involved in the Fall of the Republic, slavery and its effects rippled throughout every aspect of that turbulent time period.
Not only did slavery help push the Roman lower classes into organized mobs, but the slaves themselves understandably revolted against oppression. The 3 servile wars in the 2nd and 1st centuries BC, with the rebellion of Spartacus in the 70's BC the most notable, showed that the social system was dangerous and unhealthy. By the end of these civil wars and general social disorder, slaves were abundantly present in Rome. The slave population was at least equal to that of freedmen (non citizens), and has been estimated at anywhere from 25 to 40% of the population of the city as a whole. One such estimate suggests that the slave population in Rome circa 1 AD, may have been as much as 300,000 to 350,000 of the 900,000 total inhabitants. In outlying provinces, the numbers are certainly far less substantial, dropping to between an estimated 2 and 10% of the total. Still though, in some places such as Pergamum on the western coast of present day Turkey, the slave population may have been around 40,000 people or 1/3 of the cities total population. At the height of the Empire in the mid second century AD, some have estimated that the total slave population may have approached 10 million people, or approximately 1/6 of the population as a whole.
In the ancient world, slaves were taken simply based upon need or want. There was no ethnic or territorial preference for the taking of slaves. As the vast majority was captured as the result of Roman wars, wherever there were Roman victories, there would be new slaves. There is no evidence to suggest that the Romans placed any preference for slavery, or exceptions, based on race or country of origin. The only thing the Romans held in deference was whether or not someone was a Roman. By the mid to late imperial period, citizenship was a rather non-exclusive status, and ethnicity played little part. They were rounded up first from among the Italian tribes, where it spread to Carthage, Greece, Macedonia, Gaul and all over the eastern provinces, with little regard for origin. The Romans simply needed to replenish the stock, and the legions provided the means to do so. As examples; at the end of the Third Macedonian War in 168 BC, it was recorded that as many as 150,000 residents of Epirus were sold in Roman bondage. It's also been estimated that Julius Caesar, upon his conquest of Gaul, may have captured and enslaved 500,000 people.
Though ethnicity seems to have played little role in who would be Roman slaves, it did seem to play a part in what tasks they would be assigned to once in service. Obviously, the era one looks at will play a role, as each major conquest would bring a new influx of people from various parts of the world, but certain factors seem to hold true throughout Roman history. Gauls, Germanics and other 'barbarian' races were preferred for their strength and endurance. In fact, the Romans in many cases preferred to use these tribes in auxilia army roles rather than as slaves in the strictest sense. Still, these people were often relegated to the menial labor tasks of mining, farming and other labor related industries, reflecting upon stereotypes of the day. Greeks were especially prized slaves for both their cultural refinement and education. Greeks with the ability to educate the Roman youth or with knowledge of medicine were expensive and highly sought after. By the late empire, the predominant house slaves in Rome came almost entirely from the east (and all its various ethnicities), as Western Europe and Africa were almost exclusively of citizen class.
Roman slaves were treated in a wide variety of manners, as would be expected, depending on the circumstances, the household and the time period. Obviously, life working in a mine as a Roman slave wouldn't be desired, by contrast to that of some house slaves. Some were so highly regarded that they were considered parts of families. Tombs and gravesites lend evidence to support the praise that some Romans felt towards their slaves. Some really worked what we might consider a regular shift and were free to come and as they pleased outside of that time. Others lived in the cruelest and harshest conditions, victim to the whims of society or the cruelty of their masters. In the late Republic, slaves were strictly seen as property by the vast majority, especially at a time when the availability of new 'property' was coming in at alarming numbers. Varro called them 'vocal agricultural implements' and likely would've preferred them without the vocal part. Cato the Elder, the great politician of 'Carthage must be destroyed' fame, once suggested that old and worn out slaves be sold, as a matter of economy.
Slaves, however, could be extraordinarily expensive, and the Roman household slave certainly had a different fate. The price for a male slave in Rome at the time of Augustus has been quoted at 500 denarii. A female could go for as much as 6,000 denarii. One recorded price in Pompeii at 79 AD indicates that a slave sold for 2,500 sestertii or 625 denarii. The expense of slaves made it lucrative for the smart Roman to treat them well and keep them healthy. Even in the case of gladiators, which is often misrepresented historically to show a non stop flow of blood and Roman decadence, it was considered a horrible disaster to lose a Gladiator to death or career ending injury. These slaves were worth their weight in gold, and while still kept closely guarded, they could also be afforded the greatest of luxuries when appropriate. Great fame and fortune could not only come to the owner, but the gladiators as well, and the best of the best were treated as such. Some Romans would even sell themselves into slavery, including the arena, in order to pay off tremendous debts or in an effort to become famous.
There were a number of Roman laws regarding slavery, and these too, changed over time. In the Republican period, as already suggested, slaves had no rights and were always subject to the whims of their owners. They did have some legal standing, however. They were allowed to act as witnesses in trials, and could gain freedom either through their owner's gratitude after loyal service or by buying it through the meager earnings they might collect over a lifetime of service. For example, owners in the Republic had the right to kill or mutilate slaves at a whim, but later imperial laws took this right away, though in practice this law could be largely ignored.
As the empire changed, and social conditions along with it, the spread of slavery slowed and eventually was transformed. The Christian church and its policies regarding bondage helped alter the conditional mindset of the populace, despite the fact that it and its priest often owned slaves as well. More importantly perhaps than the religious notions, however, were the economic and even military conditions of the time. As Roman military objectives were altered from one of conquest to border defense, the continual mass influx of new slave labor ceased. The cost to purchase slaves along with a completely destablized economy made employment of the free masses at cheap wages a far more attractive alternative. The shift from central Roman Imperial power to local lords, kings and feudalism brought about a new condition of serf or peasant labor where the masses were not necessarily owned slaves but were tied directly to the land owned by these local lords. While in theory this evolution from ancient slavery to middle age European serfdom may have been more attractive, the conditions of the time and the drastically limited personal opportunities may have been far worse, or at least no better than the ancient Roman form of slavery.