At Rome a slave could not legally own property, since legally speaking only the pater familias and women who were sui iuris (a rare state) could legally have ownership. Both slaves and children in potestate, however, could "own" a peculium. The peculium consisted of assets that, while legally owned by the master, were administered entirely by the son in potestate or by the slave to which they were entrusted. These assets might consist of money, or they might consist of property, other slaves, or even just administration over businesses or other properties. Once in a slave's or son's peculium all transactions involving these assets were more or less entirely out of the hands of the father, although he was legally still liable (up to a certain amount) for them. But he didn't have to approve or even know about how the peculium was being used, although he could legally revoke administration of those assets at any time, since legally it was his property.

In practice the peculium was the property of the slave or son to whom it was entrusted, though legally it was owned by the pater familias. Household slaves, rather than agricultural or other laborers, had more opportunity to secure peculia for themselves and more chances to actually enter into business with it, but a slave had to be given a peculium in the first place to buy his freedom, and far from every household slave was entrusted with peculia. Generally the idea behind the peculium was that the slave would, through transactions and other business, increase the value of the master's assets. Having earned a certain amount for his master he might offer his master the increased peculium, thereby legally paying off his own slavery and becoming a statuliber. This does not mean, however, that any slave with a peculium could buy his freedom. Masters had to give their permission to a slave trying to buy his freedom, and if you were a jerkoff you could just continually forbid your slave from buying his freedom.