Girls between the ages of six and ten, originally from patrician and later from any freeborn family were eligible to become Vestals (sacerdotes Vestales) provided they met certain criteria, including being free of bodily imperfection and having living parents. From those offered, the selections were made by lot. In exchange for a commitment of thirty years (ten in training, ten in service, and ten training others) and a vow of chastity, Vestals were emancipated and so, free to administer their own affairs without a guardian (that is, they were free of their father's potestas), given honor, the right to make a will, luxurious accommodations at state expense, and when they went out, fasces were carried before them. They wore distinctive dress and the hairstyle of a Roman bride.

The Vestals' chief function was preservation of an undying fire (ignis inextinctus) in the shrine of Vesta, goddess of the hearth, but they had other functions as well. On May 15, the Vestals threw straw figurines (Argei) into the Tiber. At the beginning of the June Vestalia festival, the inner sanctum (penus) of the circular shrine to Vesta, in the forum Romanum, was opened for women to bring offerings; otherwise it was closed to all but the Vestals and the Pontifex Maximus. The Vestals made holy cakes (mola salsa) for the Vestalia, according to ritual prescriptions, from special salt, water and grain. On the last day of the festival, the temple was ritually cleansed. The Vestals also kept wills and participated in ceremonies.

The Vestals weren't the only priestly office Numa Pompilius instituted. Among others, he created the office of Pontifex Maximus to preside over rites, prescribe rules for public ceremony, and watch over the Vestals. It was the Pontifex' task to administer their punishment. For some offenses, a Vestal might be whipped, but if the sacred fire went out, it proved a Vestal was impure. Her impurity threatened the safety of Rome. A Vestal who lost her virginity was buried alive in the Campus Sceleratus (near the Colline gate) amid solemn ritual. The Vestal was brought to steps leading down to a room with food, a bed, and a lamp. After her descent, the steps were removed and dirt heaped on the entrance to the room. There she was left to die.