Optimates (/ˌɒptɪˈmeɪtiːz/, /ˈɒptɪmeɪts/; Latin for "best ones"; sg. optimas) and populares (/ˌpɒpjʊˈlɛəriːz, -jə-, -ˈleɪriːz/; Latin for "supporters of the people"; sg. popularis) are labels applied to politicians, political groups, traditions, strategies, or ideologies in the late Roman Republic. There is "heated academic discussion" as to whether Romans would have recognised an ideological content or political split in the label.

Among other things, optimates have been seen as supporters of the continued authority of the senate, politicians who operated mostly in the senate, or opponents of the populares. The populares have also been seen as focusing on operating before the popular assemblies, generally in opposition to the senate, using "the populace, rather than the senate, as a means [for advantage]". References to optimates (also called boni, "good men") and populares are found among the writings of Roman authors of the 1st century BC. The distinction between the terms is most clearly established in Cicero's Pro Sestio, a speech given and published in 56 BC, where he framed the two labels against each other.

With the publication of the Römische Geschichte in the 1850s, the German historian Theodor Mommsen set the enduring and popular interpretation that optimates and populares represented political parties, which he implicitly compared to the German liberal and conservative parties of his own day. Mommsen's paradigm, however, has been criticised by generations of historians, first by Friedrich Münzer, followed by Ronald Syme, who considered that Roman politics was marked by familial and individual ambitions, not parties. Other historians have pointed to the impossibility of applying such labels to many individuals, who could pretend to be popularis or optimas as they saw fit; the careers of Drusus or Pompey are for example impossible to fit into one "party". Ancient usage was also far from clear: even Cicero, while linking optimates to Greek aristokratia (ἀριστοκρατία), also used the word populares to describe politics "completely compatible with... honourable aristocratic behaviour".

As a result, modern historians do not recognise any "coherent political party" under either populares or optimates, nor do those labels lend themselves easily to comparison with a modern left–right split. Democratic interpretations of Roman politics, however, have pushed for a re-evaluation which attributes an ideological tendency – e.g. populares believing in popular sovereignty – to the labels.

With the publication of the Römische Geschichte in the 1850s, the German historian Theodor Mommsen set the enduring and popular interpretation that optimates and populares represented aristocratic and democratic parliamentary-style political parties, with the labels emerging around the time of the Gracchi. His interpretation "owe[d] much to nineteenth century German liberal thought". Classicists today, however, generally agree that neither optimate or popularis referred to political parties: "It is common knowledge nowadays that populares did not constitute a coherent political group or 'party' (even less so than their counterparts, optimates)".

Unlike in modern times, Roman politicians stood for office on the basis of their personal reputations and qualities rather than with a party manifesto or platform. For example, the opposition to the First Triumvirate failed to act as a united front with coherent coordination of its members, acting instead on an ad hoc basis with regular defections to and from those opposing the political alliance depending on the topic of debate, personal relations, etc. These ad hoc alliances and many different methods of gaining political influence meant there were no "neat categories of optimates and populares" or of conservatives and radicals in a modern sense. Erich S Gruen, for example, in Last Generation of the Roman Republic (1974) rejected both populares and optimates, saying "such labels obscure rather than enlighten" and arguing that optimates was used not as a political label, but instead used to praise a member of the political elite.

Moving away from the 19th century view of political parties or factions vying for dominance, the scope of the modern academic debate focuses on whether the terms referred to an ideological split among aristocrats or whether the terms were meaningless or topics of debate themselves.

The traditional view of the optimates refers to aristocrats who defended their own material and political interests and behaved akin to modern fiscal conservatives in opposing wealth redistribution and supporting small government. To that end, the optimates were viewed traditionally as emphasising the authority or influence of the senate over other organs of the states, including the popular assemblies. In other instances, the optimates are defined "somewhat mechanically, as those who opposed the populares".