Concepts that originated in the Roman constitution live on in both forms of government to this day. Examples include checks and balances, the separation of powers, vetoes, filibusters, quorum requirements, term limits, impeachments, the powers of the purse, and regularly scheduled elections. Even some lesser used modern constitutional concepts, such as the bloc voting found in the electoral college of the United States, originate from ideas found in the Roman constitution.
Over the years, the Roman constitution continuously evolved. By the late 5th century BC, the Constitution of the Roman Kingdom had given way to the Constitution of the Roman Republic. By 27 BC, the Constitution of the Roman Republic had transformed into the Constitution of the Roman Empire. By 300 AD, the Constitution of the Roman Empire had been reformed into the Constitution of the Late Roman Empire. The actual changes, however, were quite gradual. Together, these four constitutions formed four epochs in the continuous evolution of one master constitution.
The first Roman assembly, the 'comitia curiata', was founded during the early kingdom. Its only political role was to elect new kings. Sometimes, the king would submit his decrees to it for ratification. In the early years of the Republic, the comitia curiata was the only legislative assembly with any power. Shortly after the founding of the republic, however, the comitia centuriata and the comitia tributa became the predominant elective and legislative assemblies.
Most modern legislative assemblies are bodies consisting of elected representatives. Their members typically propose and debate bills. These modern assemblies use a form of representative democracy. In contrast, the assemblies of the Roman Republic used a form of direct democracy. The Roman assemblies were bodies of ordinary citizens, rather than elected representatives. In this regard, bills voted on (called plebiscites) were similar to modern popular referenda.
Unlike many modern assemblies, Roman assemblies were not bicameral. That is to say that bills did not have to pass both major assemblies in order to be enacted into law. In addition, no other branch had to ratify a bill (rogatio) in order for it to become law (lex). Members also had no authority to introduce bills for consideration; only executive magistrates could introduce new bills. This arrangement is also similar to what is found in many modern countries. Usually, ordinary citizens cannot propose new laws for their enactment by a popular election. Unlike many modern assemblies, in the early Republic, the Roman assemblies also had judicial functions.
After the founding of the empire, the vast majority of the powers of the assemblies were transferred to the Senate. When the Senate elected magistrates, the results of those elections would be read to the assemblies. Occasionally, the emperor would submit laws to the comitia tributa for ratification. The assemblies ratified laws up until the reign of the emperor Domitian. After this point, the assemblies simply served as vehicles through which citizens would organize.
The Roman senate was the most permanent of all of Rome's political institutions. It was probably founded before the first king of Rome ascended the throne. It survived the fall of the Roman Kingdom in the late 5th century BC, the fall of the Roman Republic in 27 BC, and the fall of the Roman Empire in 476 AD. It was, in contrast to many modern institutions named 'Senate', not a legislative body, but rather, an advisory one.