Being honorably discharged from the army
Enrolling into a legion.
Being born in Rome (as long as your parents were Roman).
Being free (after Caracalla's decree in 212 CE).
Could pass citizenship onto children.
Could make a will.
Could vote in popular assemblies (ONLY MEN).
Could wear a toga.
Legally protected from physical abuse without trial.
The right to use the three-name system.
Captives from other regions that Rome had conquered during their expansion.
Children of slaves, which was more beneficial because they didn't need to buy them.
Taking abandoned and exposed children from the streets.
Trading with other groups of people from other regions such as, Western Europe, India, & etc.
Some would voluntarily sell themselves or their children to slavery, often dye to poverty and fear of starvation, both of which would be less of a problem after becoming a slave.
Setting fire to their owners' property.
Committing suicide.
Commit acts of "sabotage" (neglecting crops, selling their owners' livestock)
Roman citizens or belong to a community that allow them to marry Roman citizens.
Be of minimum age (14 for boys and 12 for girls).
Not be too closely related.