The most common architectural plan had a rectangular temple raised on a high podium, with a clear front with a portico at the top of steps, and a triangular pediment above columns. The sides and rear of the building had much less architectural emphasis, and typically no entrances. There were also circular plans, generally with columns all round, and outside Italy there were many compromises with traditional local styles. The Roman form of temple developed initially from Etruscan temples, themselves influenced by the Greeks, with subsequent heavy direct influence from Greece.
Public religious ceremonies of the official Roman religion took place outdoors, and not within the temple building. Some ceremonies were processions that started at, visited, or ended with a temple or shrine, where a ritual object might be stored and brought out for use, or where an offering would be deposited. Sacrifices, chiefly of animals, would take place at an open-air altar within the templum; often on one of the narrow extensions of the podium to the side of the steps. Especially under the Empire, exotic foreign cults gained followers in Rome, and were the local religions in large parts of the expanded Empire. These often had very different practices, some preferring underground places of worship, while others, like Early Christians, worshipped in houses.
Some remains of many Roman temples survive, above all in Rome itself, but the relatively few near-complete examples were nearly all converted to Christian churches (and sometimes subsequently to mosques), usually a considerable time after the initial triumph of Christianity under Constantine. The decline of Roman religion was relatively slow, and the temples themselves were not appropriated by the government until a decree of the Emperor Honorius in 415. Santi Cosma e Damiano, in the Roman Forum, originally the Temple of Romulus, was not dedicated as a church until 527. The best known is the Pantheon, Rome, which is however highly untypical, being a very large circular temple with a magnificent concrete roof, behind a conventional portico front.
The English word "temple" derives from the Latin templum, which was originally not the building itself, but a sacred space surveyed and plotted ritually. The Roman architect Vitruvius always uses the word templum to refer to the sacred precinct, and not to the building. The more common Latin words for a temple or shrine were sacellum (a small shrine or chapel), aedes, delubrum, and fanum (in this article, the English word "temple" refers to any of these buildings, and the Latin templum to the sacred precinct).
The form of the Roman temple was mainly derived from the Etruscan model, but in the late Republic there was a switch to using Greek classical and Hellenistic styles, without much change in the key features of the form. The Etruscans were a people of northern Italy, whose civilization was at its peak in the seventh century BC. The Etruscans were already influenced by early Greek architecture, so Roman temples were distinctive but with both Etruscan and Greek features. Surviving temples (both Greek and Roman) lack the extensive painted statuary that decorated the rooflines, and the elaborate revetments and antefixes, in colourful terracotta in earlier examples, that enlivened the entablature.