The populace in the early Roman Empire was composed of several groups of distinct legal standing, including the Roman citizens themselves (cives romani), the provincials (provinciales), foreigners (peregrini) and free non-citizens such as freedmen (freed slaves) and slaves.
Roman citizens were subject to the Roman legal system while provincials were subject to whatever laws and legal systems had been in place in their area at the time it was annexed by the Romans.
Over time, Roman citizenship was gradually extended more and more and there was a regular "siphoning" of people from less privileged legal groups to more privileged groups, increasing the total percentage of subjects recognized as Roman citizens (e.g. Romans) though the incorporation of the provinciales and peregrini.

The Roman Empire's capability to integrate peoples in this way was one of the key elements which ensured its success.
In Antiquity, it was significantly easier to become a Roman than it was to become a member or citizen of any other contemporary state.
This aspect of the Roman state was seen as important even by some of the emperors.
For instance, Emperor Claudius pointed it out when questioned by the senate on admitting Gauls to join the senate:
What else proved fatal to Lacedaemon or Athens, in spite of their power in arms, but their policy of holding the conquered aloof as alien-born?
But the sagacity of our own founder Romulus was such that several times he fought and naturalized a people in the course of the same day!

From the Principate onwards, "barbarians" (peoples from beyond Rome's borders) settled and integrated into the Roman world.
Such settlers would have been granted certain legal rights simply by being within Roman territory, becoming provinciales and thus being eligible to serve as auxilia (auxiliary soldiers), which in turn made them eligible to become full cives Romani.
Through this relatively rapid process, thousands of former barbarians could quickly become Romans.
This tradition of straightforward integration eventually culminated in the Antonine Constitution, issued by Emperor Caracalla in 212, in which "all the people of the Roman world" (e.g. provinciales, peregrini and freedmen alike) were formally granted Roman citizenship.
At this point, Roman citizenship in the empire was not as significant as it had been in the republic, chiefly due to the change from a republican to an imperial government invalidating the need for voting rights and because service in the Roman military was no longer compulsory.
Caracalla's grant contributed to a vast increase in the number of people with the nomen (name indicating familial association) Aurelius (Caracalla was a nickname for the emperor, whose actual name was Marcus Aurelius Severus Antoninus).

By the time of Caracalla's edict, there were already many people throughout the provinces who were considered (and considered themselves) Romans; through the centuries of Roman expansion large numbers of veterans and opportunists had settled in the provinces.
Colonies founded by Julius Caesar and Augustus alone saw between 500.000 and a million people from Italy settled in Rome's provinces.
Around the time of Augustus's death, four to seven percent of the free people in the provinces of the empire were Roman citizens.
In addition to colonists, many provincials had also become citizens through grants by emperors (who sometimes granted citizenship to individuals, families or cities), holding offices in certain cities or serving in the army.
The populace in the early Roman Empire was composed of several groups of distinct legal standing, including the Roman citizens themselves (cives romani), the provincials (provinciales), foreigners (peregrini) and free non-citizens such as freedmen (freed slaves) and slaves. Roman citizens were subject to the Roman legal system while provincials were subject to whatever laws and legal systems had been in place in their area at the time it was annexed by the Romans. Over time, Roman citizenship was gradually extended more and more and there was a regular "siphoning" of people from less privileged legal groups to more privileged groups, increasing the total percentage of subjects recognized as Roman citizens (e.g. Romans) though the incorporation of the provinciales and peregrini.
The Roman Empire's capability to integrate peoples in this way was one of the key elements which ensured its success. In Antiquity, it was significantly easier to become a Roman than it was to become a member or citizen of any other contemporary state. This aspect of the Roman state was seen as important even by some of the emperors. For instance, Emperor Claudius pointed it out when questioned by the senate on admitting Gauls to join the senate:
What else proved fatal to Lacedaemon or Athens, in spite of their power in arms, but their policy of holding the conquered aloof as alien-born? But the sagacity of our own founder Romulus was such that several times he fought and naturalized a people in the course of the same day!
From the Principate onwards, "barbarians" (peoples from beyond Rome's borders) settled and integrated into the Roman world. Such settlers would have been granted certain legal rights simply by being within Roman territory, becoming provinciales and thus being eligible to serve as auxilia (auxiliary soldiers), which in turn made them eligible to become full cives Romani. Through this relatively rapid process, thousands of former barbarians could quickly become Romans. This tradition of straightforward integration eventually culminated in the Antonine Constitution, issued by Emperor Caracalla in 212, in which "all the people of the Roman world" (e.g. provinciales, peregrini and freedmen alike) were formally granted Roman citizenship. At this point, Roman citizenship in the empire was not as significant as it had been in the republic, chiefly due to the change from a republican to an imperial government invalidating the need for voting rights and because service in the Roman military was no longer compulsory. Caracalla's grant contributed to a vast increase in the number of people with the nomen (name indicating familial association) Aurelius (Caracalla was a nickname for the emperor, whose actual name was Marcus Aurelius Severus Antoninus).
By the time of Caracalla's edict, there were already many people throughout the provinces who were considered (and considered themselves) Romans; through the centuries of Roman expansion large numbers of veterans and opportunists had settled in the provinces. Colonies founded by Julius Caesar and Augustus alone saw between 500.000 and a million people from Italy settled in Rome's provinces. Around the time of Augustus's death, four to seven percent of the free people in the provinces of the empire were Roman citizens. In addition to colonists, many provincials had also become citizens through grants by emperors (who sometimes granted citizenship to individuals, families or cities), holding offices in certain cities or serving in the army.
Roman people