in ancient Republican Rome, a legal assembly of the people. Comitia met on an appropriate site (comitium) and day (comitialis) determined by the auspices (omens). Within each comitia, voting was by group; the majority in each group determined its vote.

The powers of Republican Roman government were divided between the Senate, the magistrates, and the people (populus). Originally the populus consisted only of the patricians, who formed a class of privileged citizens. The patricians were divided into 30 curiae, or local groups, and the legal assembly of these curiae, the Comitia Curiata, was for a time the sole legal representative of the entire Roman populus. The Comitia Curiata dated from the time of the Roman kings. By late Republican times its importance had dwindled, and its chief functions were simply to confer the imperium (supreme executive power) on magistrates and to witness wills, adoptions, and the inauguration of priests.

The Comitia Centuriata, instituted in about 450 BC as a military assembly, decided issues of war and peace, enacted legislation, elected consuls, praetors, and censors, and considered the appeals of Roman citizens convicted of capital crimes. Unlike the Comitia Curiata, this comitia included plebeians as well as patricians, but its organization nevertheless gave greater influence to the rich than to the poor. All Roman citizens were registered in tribus (tribes (tribe)), and a census was made of their property. They were then assigned to classes and centuriae (centuries) according to their wealth and the equipment they could provide for military service. Voting in the Comitia Centuriata proceeded by centuries according to precedence, starting with the equites, followed by the first and wealthiest class; these groups constituted a clear majority over the combined votes of the other four classes if they voted as a block.

In 471 BC the Concilium Plebis was established; this body was organized and voted by tribes, and it consisted exclusively of plebeians and could be summoned and presided over only by the plebeian magistrates, i.e., the tribunes. The Concilium Plebis was originally a relatively small and informal advisory assembly, or concilium, but after the passage of the Lex Hortensia (287 BC) its resolutions, or plebiscita, had the force of law and were binding upon all Roman citizens. The assembly became, in effect, the Comitia Plebis Tributa. Its simpler procedures and the availability of tribunes made this comitia an important legislative body of the middle and later periods of Republican Rome. Its judicial functions, however, were basically limited to fines for noncapital offenses.

The Comitia Populi Tributa was founded around 357 BC in imitation of the Comitia Plebis Tributa, but it differed from the former in that it was an assembly of the whole Roman people, plebeians and patricians, who were organized by tribe. This comitia elected the minor magistrates (curule aediles, quaestors, and military tribunes), held minor trials, and eventually became a regular organ for laws passed by the whole people. Both the Comitia Plebis Tributa and the Comitia Populi Tributa became increasingly influenced by radical tribunes or other demagogic leaders from the period of the Gracchi (c. 130 BC) onward.

Imperial Roman territory extended too widely for more than a few citizens to attend the comitia from distant regions. In spite of the emperor Augustus' provision for local councillors to vote for Roman comitia in their own towns, the comitia began to decline, and the various elective, legislative, and judicial functions gradually lapsed under the principate. The last piece of recorded legislation by the comitia is an agrarian law carried by the emperor Nerva in AD 98.

There were also comitia in municipia and coloniae for the election of magistrates and the passage of local legislation, but these comitia also decayed under the empire.