The proletarii constituted a social class of Roman citizens owning little or no property. The origin of the name is presumably linked with the census, which Roman authorities conducted every five years to produce a register of citizens and their property from which their military duties and voting privileges could be determined. For citizens with property valued 11,000 asses or less, which was below the lowest census for military service, their children—proles (from Latin prōlēs, "offspring")—were listed instead of their property; hence, the name proletarius, "the one who produces offspring". The only contribution of a proletarius to the Roman society was seen in his ability to raise children, the future Roman citizens who can colonize new territories conquered by the Roman Republic and later by the Roman Empire. The citizens who had no property of significance were called capite censi because they were "persons registered not as to their property...but simply as to their existence as living individuals, primarily as heads (caput) of a family."
Although included in one of the five support centuriae of the Comitia Centuriata, proletarii were largely deprived of their voting rights due to their low social status caused by their lack of "even the minimum property required for the lowest class" and a class-based hierarchy of the Comitia Centuriata. The late Roman historians, such as Livy, not without some uncertainty, understood the Comitia Centuriata to be one of three forms of popular assembly of early Rome composed of centuriae, the voting units whose members represented a class of citizens according to the value of their property. This assembly, which usually met on the Campus Martius to discuss public policy issues, was also used as a means of designating military duties demanded of Roman citizens. One of reconstructions of the Comitia Centuriata features 18 centuriae of cavalry, and 170 centuriae of infantry divided into five classes by wealth, plus 5 centuriae of support personnel called adsidui. The top infantry class assembled with full arms and armor; the next two classes brought arms and armor, but less and lesser; the fourth class only spears; the fifth slings. In voting, the cavalry and top infantry class were enough to decide an issue; as voting started at the top, an issue might be decided before the lower classes voted. In the last centuries of the Roman Republic (509-44 B.C.), the Comitia Centuriata became impotent as a political body, which further eroded already minuscule political power the proletarii might have had in the Roman society.
Following a series of wars the Roman Republic engaged since the closing of the Second Punic War (218–201 B.C.), such as the Jugurthine War and conflicts in Macedonia and Asia, the significant reduction in the number of Roman family farmers had resulted in the shortage of people whose property qualified them to perform the citizenry's military duty to Rome. As a result of the Marian reforms initiated in 107 B.C. by the Roman general Gaius Marius (157–86), the proletarii became the backbone of the Roman Army.
Karl Marx, who studied Roman law at the University of Berlin, used the term proletariat in his socio-political theory of Marxism to describe a working class unadulterated by private property and capable of a revolutionary action to topple capitalism in order to create classless society.
In Marxist theory, the proletariat is the social class that does not have ownership of the means of production and whose only means of subsistence is to sell their labor power for a wage or salary. Proletarians are wage-workers, while some refer to those who receive salaries as the salariat. For Marx, however, wage labor may involve getting a salary rather than a wage per se. Marxism sees the proletariat and bourgeoisie (capitalist class) as occupying conflicting positions, since workers automatically wish their wages to be as high as possible, while owners and their proxies wish for wages (costs) to be as low as possible.