They were not the largest baths in Rome, but were by far the most luxurious. The Baths of Caracalla were begun in 206 AD by Settimio Severo and were completed by his son Caracalla in 217 AD. They were made to accommodate 2,000 bathers and many thousands more in the huge complex that accompanied the bathing areas. These baths continued to function until the Goths cut off the aqueducts that supplied the water in the 6th Century.

Four baths had already been built, when Caracalla finished his 25-acre masterpiece. The plain exterior held a constrastingly rich interior. The floors were paved with marble and mosaic, with walls of the same design. Vaulting, a practice of using large archways and cathedral like ceilings, was used extensively throughout the baths. Ninety-eight feet high in some places, the vaulting became a trademark of the baths.

The baths consisted of a central block enclosed by a wall with gateways opening into Rome's streets. The southwest side was entirely taken up by water tanks, which were screened off by a large amphitheater. On each side of the baths were pavilions containing the Latin and Greek libraries. The other large areas were probably gymnasiums where the Romans would exercise. The three main rooms, the caldarium, tepidarium, and frigidarium, were at the heart of the enclosure. Symmetrically arranged on both sides of these rooms were secondary rooms, which included changing rooms, smaller gymnasiums, and steam baths. When bathers entered, they separated into these side rooms, only to meet in the large central rooms to bath. Many rooms, including the gymnasium (palaestra) and the changing room (apodyterium), still contain the original mosaic designs. Many rooms were changed and mosaics were altered to contain a religious theme.

Romans would follow the instructions of their doctors, to go from the changing room, to the gymnasium. After working out for a while, they would go into a heated room where they would perspire for a longer time. Then the bather would continue into the caldarium or hot bath, to scrape off all impurities on their skin and swim in the hot water. The caldarium was a huge circular room covered by a dome with some supporting pillars still standing. After leaving the caldarium, the bathers would enter the tepidarium, the cooler room, before plunging into the cold waters of the frigidarium. This was a daily routine for most of ancient Rome's citizens until 537 AD.